feat(libdoc): read specversion 4 (Robot Framework 7.5)

[aaltat]

Fixes the failing Library release run for Browser 20.6.0.

Why it failed

scripts/fetch-libdoc.sh floors Robot Framework at >=7.4, so uv picked the newly released RF 7.5. Its Libdoc writes specversion: 4, and transform rejects anything but 3.

What spec 4 changes

Diffing Browser 20.6.0 generated with RF 7.4.1 and RF 7.5: no keys removed, counts identical, but

• each keyword's Arguments: list is lifted out of doc into args[].doc (144 of 152 keywords), and the return section into returnDoc
• returnType is a None node where spec 3 wrote null
• alias on type nodes and raises on keywords are new, always empty in Browser

Only bumping the version would have silently dropped every argument description from the reference.

Changes

• lib/libdoc.ts accepts specversions 3 and 4; arguments and returns carry their own sanitized, link-rewritten doc (empty for spec 3, which keeps it in the body)
• KeywordPanels.server.vue renders an argument's doc as a row under it, and the return doc under Returns
• a None return type means no return value, in the transform and in keyword-diff.ts (which would otherwise report None → None)
• the build's shrink guard counts argument and return docs too

Verified

• tests for spec 4, spec 3 and the None diff case written first and seen failing; full suite passes (732)
• generated 20.6.0 locally with RF 7.5 and built the site with it as latest: all 524 argument docs render, links resolve, keyword diff 20.4.0 → 20.6.0 shows no changed signatures; checked desktop, mobile and dark mode
• the 20.6.0 libdoc itself is not committed: once this merges, the next scheduled run imports it with the release notes, contributors and versions

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