Escape trailing placeholders in rule documentation

[charliermarsh]

Summary

If a rule ends with a trailing placeholder (like "Use {target}"), that gets interpreted as an HTML attribute adding, target="target" to the node. This PR escapes such cases. In reality, they're rare, since we almost always wrap placeholders in backticks, which avoids this problem -- but in some cases, they are in fact correct to be un-backticked.

Closes #9288.

Test Plan

[github-actions]

ruff-ecosystem results

Formatter (stable)

ℹ️ ecosystem check encountered format errors. (no format changes; 2 project errors)

pypa/setuptools (error)

ruff failed
  Cause: 'quote-style = preserve' is a preview only feature. Run with '--preview' to enable it.

sphinx-doc/sphinx (error)

warning: The following rules may cause conflicts when used with the formatter: `COM812`. To avoid unexpected behavior, we recommend disabling these rules, either by removing them from the `select` or `extend-select` configuration, or adding them to the `ignore` configuration.
warning: The `flake8-quotes.inline-quotes="single"` option is incompatible with the formatter's `format.quote-style="double"`. We recommend disabling `Q000` and `Q003` when using the formatter, which enforces a consistent quote style. Alternatively, set both options to either `"single"` or `"double"`.
warning: Detected debug build without --no-cache.
error: Failed to read tests/roots/test-pycode/cp_1251_coded.py: stream did not contain valid UTF-8

Formatter (preview)

ℹ️ ecosystem check encountered format errors. (no format changes; 1 project error)

sphinx-doc/sphinx (error)

ruff format --preview

warning: The following rules may cause conflicts when used with the formatter: `COM812`. To avoid unexpected behavior, we recommend disabling these rules, either by removing them from the `select` or `extend-select` configuration, or adding them to the `ignore` configuration.
warning: The `flake8-quotes.inline-quotes="single"` option is incompatible with the formatter's `format.quote-style="double"`. We recommend disabling `Q000` and `Q003` when using the formatter, which enforces a consistent quote style. Alternatively, set both options to either `"single"` or `"double"`.
warning: Detected debug build without --no-cache.
error: Failed to read tests/roots/test-pycode/cp_1251_coded.py: stream did not contain valid UTF-8