📝 docs(migration): slot html5validator by download rank

[gaborbernat]

The html5validator migration guide was appended to the end of its slot when it landed in #599, so it sorted below libraries with far fewer users. Its 33,804 monthly PyPI downloads put it just under calmjs.parse (44,251), so this moves it there in both places the guides are ordered by adoption: the hidden toctree (between calmjs-parse and html5-parser in the global download ranking) and the visible per-namespace list-table, where it now sits right after calmjs.parse in the Clean group with the trailing rank numbers shifted down one. No badge URLs change and no other guide's relative order moves.

[codspeed-hq]

Merging this PR will not alter performance

✅ 87 untouched benchmarks
⏩ 18 skipped benchmarks¹

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_{Comparing gaborbernat:docs/reslot-html5validator (ec425e8) with main (917a854)}

Footnotes

1. 18 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩