doc: Document markup syntax

Not depicted:

- Diagrams using Lua filter and dot
- fenced code attributes (enabled by the attributes extension)

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doc/contributing/contributing-to-documentation.chapter.md
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# Contributing to this documentation {#chap-contributing}
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The DocBook sources of the Nixpkgs manual are in the [doc](https://github.com/NixOS/nixpkgs/tree/master/doc) subdirectory of the Nixpkgs repository.
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The sources of the Nixpkgs manual are in the [doc](https://github.com/NixOS/nixpkgs/tree/master/doc) subdirectory of the Nixpkgs repository. The manual is still partially written in DocBook but it is progressively being converted to [Markdown](#sec-contributing-markup).
You can quickly check your edits with `make`:
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```
If the build succeeds, the manual will be in `./result/share/doc/nixpkgs/manual.html`.
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## Syntax {#sec-contributing-markup}
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As per [RFC 0062](https://github.com/NixOS/rfcs/pull/72), all new documentation content should be written in [CommonMark](https://commonmark.org/) Markdown dialect.
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Additionally, the following syntax extensions are currently used:
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- []{#ssec-contributing-markup-anchors}
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Explicitly defined **anchors** on headings, to allow linking to sections. These should be always used, to ensure the anchors can be linked even when the heading text changes, and to prevent conflicts between [automatically assigned identifiers](https://github.com/jgm/commonmark-hs/blob/master/commonmark-extensions/test/auto_identifiers.md).
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It uses the widely compatible [header attributes](https://github.com/jgm/commonmark-hs/blob/master/commonmark-extensions/test/attributes.md) syntax:
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```markdown
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## Syntax {#sec-contributing-markup}
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```
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- []{#ssec-contributing-markup-anchors-inline}
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**Inline anchors**, which allow linking arbitrary place in the text (e.g. individual list items, sentences…).
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They are defined using a hybrid of the link syntax with the attributes syntax known from headings, called [bracketed spans](https://github.com/jgm/commonmark-hs/blob/master/commonmark-extensions/test/bracketed_spans.md):
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```markdown
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- []{#ssec-gnome-hooks-glib} `glib` setup hook will populate `GSETTINGS_SCHEMAS_PATH` and then `wrapGAppsHook` will prepend it to `XDG_DATA_DIRS`.
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```
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- []{#ssec-contributing-markup-admonitions}
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**Admonitions**, set off from the text to bring attention to something.
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It uses pandoc’s [fenced `div`s syntax](https://github.com/jgm/commonmark-hs/blob/master/commonmark-extensions/test/fenced_divs.md):
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```markdown
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::: {.warning}
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This is a warning
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:::
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```
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which renders as
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> ::: {.warning}
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> This is a warning.
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> :::
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The following are supported:
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- [`caution`](https://tdg.docbook.org/tdg/5.0/caution.html)
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- [`important`](https://tdg.docbook.org/tdg/5.0/important.html)
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- [`note`](https://tdg.docbook.org/tdg/5.0/note.html)
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- [`tip`](https://tdg.docbook.org/tdg/5.0/tip.html)
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- [`warning`](https://tdg.docbook.org/tdg/5.0/warning.html)
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- []{#ssec-contributing-markup-definition-lists}
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[**Definition lists**](https://github.com/jgm/commonmark-hs/blob/master/commonmark-extensions/test/definition_lists.md), for defining a group of terms:
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```markdown
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pear
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: green or yellow bulbous fruit
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watermelon
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: green fruit with red flesh
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```
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which renders as
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> pear
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> : green or yellow bulbous fruit
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>
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> watermelon
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> : green fruit with red flesh
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For contributing to the legacy parts, please see [DocBook: The Definitive Guide](https://tdg.docbook.org/) or the [DocBook rocks! primer](https://web.archive.org/web/20200816233747/https://docbook.rocks/).