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/authentication.rst

@@ -1093,7 +1093,7 @@ All three endpoints support both HTML and JSON responses. Visit the endpoint dir
 .. _PermissionRulesView:
 
 Permission rules view
-======================
+=====================
 
 The ``/-/rules`` endpoint displays all permission rules (both allow and deny) for each candidate resource for the requested action.
 
@@ -1106,7 +1106,7 @@ Pass ``?action=`` as a query parameter to specify which action to check.
 .. _PermissionCheckView:
 
 Permission check view
-======================
+=====================
 
 The ``/-/check`` endpoint evaluates a single action/resource pair and returns information indicating whether the access was allowed along with diagnostic information.
 
@@ -1212,7 +1212,7 @@ Default *allow*.
 .. _permissions_view_database_download:
 
 view-database-download
------------------------
+----------------------
 
 Actor is allowed to download a database, e.g. https://latest.datasette.io/fixtures.db