apidoc: allow configuring template directories

[okarachidera]

Purpose

• Add apidoc_template_dir so projects can set a default template location for apidoc-generated files, and allow each apidoc_modules entry to override it with its own template_dir.
• Update the docs/CHANGES so users know about the new knobs.
• Add a regression test + fixture that checks the default vs per-module templates end up in the generated .rst output.

Local testing: blocked in this environment; tox -e py313 -- tests/test_extensions/test_ext_apidoc.py -k template_dir_option is the command I intend to run/confirm once network package installs are available.

References

• n/a

[okarachidera]

@tk0miya is failing in test_numfig_disabled_warn. The warning isn’t emitted when the cached build is reused, so the assertion sees an empty warning buffer. That’s unrelated to this change (it only touches apidoc template handling). Could someone rerun the workflow? I can’t trigger it myself.