Fix links to filtered Docutils system messages

[Michael-WhiteCapData]

Remove problematic markup links when their system message is filtered, preserving the displayed text and unregistering removed message IDs. Include detached Docutils messages when warnings are retained, with section IDs for Sphinx's HTML writer.

Fixes #13698.

Regression tests exercise unmatched emphasis and an unknown reference with keep_warnings both enabled and disabled. All 122 markup and HTML builder tests pass, along with targeted mypy, Ruff checks, formatting, and the 20 JavaScript browser specs run by the push hook.

[Michael-WhiteCapData]

CI follow-up: the standard Python jobs and static checks pass. The Docutils HEAD job reports HTML parsing failures; I reproduced the same mismatched tag: line 139, column 2 on unchanged upstream b04a2101295ac3fb725b16111eda0284b6da4cca with Docutils 35b59ca7d0645e6d7c5581a34b75f6a0feb799cc. The baseline markup subset has 64 failures and 4 passes. The LaTeX job also installs that Docutils development revision and reports the same HTML failure among additional failures.

The Python 3.13t and 3.14t jobs fail in the unchanged test_cfunction expectation: time.asctime reports time_tuple, while the test expects tuple. My available Windows 3.13.15t baseline passes that test, so this is not a claim to have reproduced the hosted 3.13.16 environment locally. I have not exhaustively reproduced every LaTeX job failure.

Additional baseline verification: on unchanged upstream b04a210, Python 3.14.7 on Windows with development Docutils reproduces four representative failures: test_footnote_referred_multiple_times (LaTeX), test_texinfo, test_lineblock, and test_highlight_language_default (doctest). With Docutils 0.22.3, the same selection gives 3 passed and 1 skipped (Texinfo requires an unavailable external tool). This extends the baseline evidence without claiming that every hosted LaTeX failure has been reproduced.