Remove obsolete hook_extension() and hook_render_template() documentation

Problem/Motivation

theme.api.php still documents hook_extension(), a hook that Drupal 11.3.0 made obsolete. #1685492: Convert theme engines into services converted theme engines from .engine files into services tagged theme_engine, implementing ThemeEngineInterface. The ENGINE_extension() naming convention and the file-inclusion mechanism that would have called it no longer exist anywhere in core.

The function is not dead code in the sense of being unreachable at runtime — it was never really "called" as a conventional hook to begin with, it was a naming pattern the old .engine-file loader looked for. It is dead documentation: it tells anyone writing a new theme engine to implement a hook that nothing in current core invokes, instead of pointing them at ThemeEngineInterface and the theme_engine service tag that TwigThemeEngine actually uses.

Steps to reproduce

1. Read core/lib/Drupal/Core/Render/theme.api.php on Drupal 11.3.0 or later (verified against main at commit eaba66f4, lines 810–822).
2. Find hook_extension(), documented as "Declare a template file extension to be used with a theme engine," in the ENGINE_extension() naming format:
   

   function hook_extension(): string {
     // Extension for template base names in Twig.
     return '.html.twig';
   }
   

3. Compare with how TwigThemeEngine actually declares its extension today — a private class constant on the service, not a hook:
   

   class TwigThemeEngine implements ThemeEngineInterface {
     protected const string EXTENSION = '.html.twig';
   }
   

4. Search core and contrib for any caller of an *_extension() function following this hook's naming convention: none exists.

Proposed resolution

Remove hook_extension() (theme.api.php, lines 810–822 at eaba66f4) and its docblock entirely. No deprecation period is needed: the hook is not invoked anywhere in core, and the mechanism that would have invoked it was already removed by #1685492: Convert theme engines into services. A follow-up documentation improvement (out of scope here) could add a short pointer from the theming/engine documentation to ThemeEngineInterface for anyone who lands on the old hook via search.

Remaining tasks

• Confirm (grep is sufficient) that no core or contrib code defines an *_extension() function relying on the old hook invocation.
• Write the patch/MR removing the function and its docblock.
• Review.

User interface changes

None.

Introduced terminology

None.

API changes

Removes the documented, but already non-functional, hook_extension(). No runtime behavior changes, since nothing in current core invokes this hook.

Data model changes

None.

Release notes snippet

Removed the obsolete hook_extension(), non-functional documentation left over from the pre-11.3 theme engine file system.