ci(doc-linter): exclude docs/_rst_legacy/ and docs/_build/ from lint scope

`is_docs_path()` returned True for any path under `docs/`, including
the archived RST shadows under `docs/_rst_legacy/` and the build
output under `docs/_build/`. Sphinx excludes both from the build
(per `docs/conf.py`'s exclude_patterns) and AGENTS marks
`_rst_legacy` as reference-only. The linter shouldn't process either.

Add a `DOCS_EXCLUDED_SUBDIRS = ('_build', '_rst_legacy')` constant.
After confirming a path is under `docs/`, walk each excluded subtree
and reject the path if it's contained.

Also unify the auto-discover walk fallback to call `is_docs_path()`
for the filter — previously it had its own hand-rolled `"_build"
not in path` check that didn't handle `_rst_legacy` at all and would
have walked the entire legacy archive. Prune `dirs[:]` in-place at
each walk level so we don't descend into the excluded subtrees in
the first place — optimization on top of correctness. Reverted the
`_dirs` -> `dirs` rename here because we now mutate it.

Test coverage: 10 hand-coded `is_docs_path()` cases — all pass:
- `docs/configuration/foo.md` -> True
- `docs/_rst_legacy/foo.rst` -> False (was True)
- `docs/_rst_legacy/subdir/rst-foo.rst` -> False (was True)
- `docs/_build/html/index.html` -> False (was True)
- `docs/_include/foo.txt` -> True (live snippets stay in scope)
- `docs` -> True
- `AGENTS.md`, `README.md`, `.github/copilot-instructions.md`,
  `scripts/doc-linter.py` -> False (already correct)

Tracked as item 4 of the rolling-side cleanup backlog from PR #2014
/ #2019 / #2020 reviews.

🤖 Generated by [robots](https://vyos.io)

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