doc/packages/lhapdf: add documentation (#341754)

doc/packages/index.md

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 kakoune.section.md
 krita.section.md
 linux.section.md
+lhapdf.section.md
 locales.section.md
 etc-files.section.md
 nginx.section.md

doc/packages/lhapdf.section.md

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+# LHAPDF {#lhapdf}
+
+[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.
+
+## PDF sets {#lhapdf-sets}
+
+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.
+
+### Setup hook {#lhapdf-sets-hook}
+
+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).