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/internals.rst

@@ -419,7 +419,7 @@ For legacy string/tuple based permission checking, use :ref:`datasette_permissio
 .. _datasette_ensure_permission:
 
 await .ensure_permission(action, resource=None, actor=None)
-------------------------------------------------------------
+-----------------------------------------------------------
 
 ``action`` - string
     The action to check. See :ref:`permissions` for a list of available actions.
@@ -1047,7 +1047,7 @@ These methods each return a ``datasette.utils.PrefixedUrlString`` object, which
 .. _internals_permission_classes:
 
 Permission classes and utilities
-=================================
+================================
 
 .. _internals_permission_sql:
 
@@ -1296,7 +1296,7 @@ Example usage:
 .. _database_execute_write:
 
 await db.execute_write(sql, params=None, block=True)
------------------------------------------------------
+----------------------------------------------------
 
 SQLite only allows one database connection to write at a time. Datasette handles this for you by maintaining a queue of writes to be executed against a given database. Plugins can submit write operations to this queue and they will be executed in the order in which they are received.
 
@@ -1313,7 +1313,7 @@ Each call to ``execute_write()`` will be executed inside a transaction.
 .. _database_execute_write_script:
 
 await db.execute_write_script(sql, block=True)
------------------------------------------------
+----------------------------------------------
 
 Like ``execute_write()`` but can be used to send multiple SQL statements in a single string separated by semicolons, using the ``sqlite3`` `conn.executescript() <https://docs.python.org/3/library/sqlite3.html#sqlite3.Cursor.executescript>`__ method.
 
@@ -1322,7 +1322,7 @@ Each call to ``execute_write_script()`` will be executed inside a transaction.
 .. _database_execute_write_many:
 
 await db.execute_write_many(sql, params_seq, block=True)
----------------------------------------------------------
+--------------------------------------------------------
 
 Like ``execute_write()`` but uses the ``sqlite3`` `conn.executemany() <https://docs.python.org/3/library/sqlite3.html#sqlite3.Cursor.executemany>`__ method. This will efficiently execute the same SQL statement against each of the parameters in the ``params_seq`` iterator, for example: