[FEATURE] Make templates overridable

[CybotTM]

Allow users to override Twig templates when rendering documentation
locally via Docker or a custom CI pipeline, without modifying the
container image.

Two methods are supported:

1. Docker volume mount at /templates (highest priority)
2. Project-bundled templates at resources/custom-templates
   (relative to the project root)

The theme extension detects these directories and prepends them to the
template search path, so custom templates take precedence over the
built-in theme templates.

Based on

This continues the work from #1085 by @linawolf, with all review
feedback addressed:

• @jaapio: Use /templates instead of /project/custom-templates
  to avoid Docker volume mount ordering issues when both paths are
  nested under /project
• @kaystrobach: Fix documentation to use --entrypoint=cat /
  --entrypoint=sh — the container entrypoint routes all commands to
  the PHP guides binary, so bare cat fails with
  "Command 'cat' is not defined"
• @kaystrobach: Implement and document the project-bundled
  resources/custom-templates path (was mentioned in original docs
  but not implemented)
• Remove duplicate "Finding the Original Templates" section
• Fix priority ordering: Docker volume templates now correctly take
  precedence over project-bundled templates

Changed files

File	Change
Dockerfile	No changes needed (Docker creates mount targets automatically)
Typo3DocsThemeExtension.php	Detect /templates and /project/resources/custom-templates with is_dir() + is_readable() guards, build template path array in priority order
Documentation/Templating/Index.rst	New documentation page covering both override methods, directory structure, and examples
Documentation/Index.rst	Add Templating to toctree

[linawolf]

@kaystrobach would this solution work for you?

[kaystrobach]

Good from reading. Only question left is how to add own logos / css etc.

we can do this after rendering with cp but this feels „hacky“ using the resources folder feels more natural for that. That would be the only missing point from my POV.

removing leftovers files is more a task for external cleanup (should not be done in the container)