Improve GitHub release rendering (#6356)

This PR hardens the release/changelog generation pipeline so ReST ->
Markdown conversion produces stable, correctly formatted links and
inline code.

- Fix broken rendering of generated references by ensuring RST
`~`-shortened forms like ``` `:class:`~beetsplug._utils...``` are
converted into real Markdown links.
- Updates `extra/release.py` to generate clearer link text for CLI
command refs (e.g., `list-cmd` becomes 'list command') while keeping the
same documentation targets.
- Simplifies the Markdown post-processing step by removing the redundant
`MD_REPLACEMENTS` layer, reducing ad-hoc formatting logic in the release
script.
- Standardizes changelog section header structure in
`docs/changelog.rst` (and the release template) to align with
Sphinx/ReST conventions and avoid ambiguous colon-based headers.
- Strengthens CI by extending `lint-docs` in `pyproject.toml` to fail on
single-backtick inline literals in `.rst`, enforcing double-backtick
literals so `pandoc` does not emit meaningless `<span
class="title-ref">...` output that breaks Markdown rendering.

Net effect: more predictable release notes output, fewer conversion edge
cases, and earlier detection of docs formatting that would render
incorrectly downstream.

Author: snejus