Add test for RST heading underline lengths, closes #2544

Added test_rst_heading_underlines_match_title_length() to verify that RST
heading underlines match their title lengths. The test properly handles:
- Overline+underline style headings (skips validation for those)
- Empty lines before underlines (ignores them)
- Minimum 5-character underline length (avoids false positives)

Running this test identified 14 heading underline mismatches which have
been fixed across 5 documentation files:
- docs/authentication.rst (3 headings)
- docs/plugin_hooks.rst (4 headings)
- docs/internals.rst (5 headings)
- docs/deploying.rst (1 heading)
- docs/changelog.rst (1 heading)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docs/deploying.rst

@@ -79,7 +79,7 @@ Datasette will not be accessible from outside the server because it is listening
 .. _deploying_openrc:
 
 Running Datasette using OpenRC
-===============================
+==============================
 OpenRC is the service manager on non-systemd Linux distributions like `Alpine Linux <https://www.alpinelinux.org/>`__ and `Gentoo <https://www.gentoo.org/>`__.
 
 Create an init script at ``/etc/init.d/datasette`` with the following contents: