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/plugin_hooks.rst

@@ -780,7 +780,7 @@ The plugin hook can then be used to register the new facet class like this:
 .. _plugin_register_permissions:
 
 register_permissions(datasette)
---------------------------------
+-------------------------------
 
 .. note::
     This hook is deprecated. Use :ref:`plugin_register_actions` instead, which provides a more flexible resource-based permission system.
@@ -830,7 +830,7 @@ The fields of the ``Permission`` class are as follows:
 .. _plugin_register_actions:
 
 register_actions(datasette)
-----------------------------
+---------------------------
 
 If your plugin needs to register actions that can be checked with Datasette's new resource-based permission system, return a list of those actions from this hook.
 
@@ -931,7 +931,7 @@ The fields of the ``Action`` dataclass are as follows:
     - Have an ``__init__`` method that accepts appropriate parameters and calls ``super().__init__(parent=..., child=...)``
 
 The ``resources_sql()`` method
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The ``resources_sql()`` classmethod is crucial to Datasette's permission system. It returns a SQL query that lists all resources of that type that exist in the system.
 
@@ -1445,7 +1445,7 @@ Example: `datasette-permissions-sql <https://datasette.io/plugins/datasette-perm
 .. _plugin_hook_permission_resources_sql:
 
 permission_resources_sql(datasette, actor, action)
----------------------------------------------------
+--------------------------------------------------
 
 ``datasette`` - :ref:`internals_datasette`
     Access to the Datasette instance.