doc/packages/lhapdf: add documentation

This documents:
* The existence of the `lhapdf` package (not particularly important)
* The existence of the `lhapdf.pdf_sets` attrset (important)
* The setup hook used in all of the entries in `lhapdf.pdf_sets`
(slightly important)

Part of #341479.

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# LHAPDF {#lhapdf}
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[LHAPDF](https://lhapdf.hepforge.org/) is a tool for evaluating parton distribution functions (PDFs) in high-energy physics. LHAPDF is available in the `lhapdf` package.
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## PDF sets {#lhapdf-sets}
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All of [the PDF sets made available by the LHAPDF project](https://lhapdf.hepforge.org/pdfsets.html) are available through the `lhapdf.pdf_sets` attrset.
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### Setup hook {#lhapdf-sets-hook}
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Each package provided in the `lhapdf.pdf_sets` attrset contains a setup hook which adds itself to [the `LHAPDF_DATA_PATH` environment variable](https://lhapdf.hepforge.org/#sets).