Add a vendor doc page for the ARPHRD crawler #2498

Summary

pcapkit/vendor/linux/arphrd.py (merged on #2497) has no matching Sphinx doc page under docs/source/pcapkit/vendor/, unlike the established precedent vendor/ipx.rst (indexed under Internet Layer) for the same DATA-attribute crawler shape.

Scope

Add docs/source/pcapkit/vendor/linux.rst (or arphrd.rst) mirroring ipx.rst's format -- a "this crawler does not crawl" note, autoclass block, and a footnote to the Linux kernel header source -- and index it under "Link Layer" in docs/source/pcapkit/vendor/index.rst.

Found while reviewing #2466, which had the same gap for its sslv2 crawler.