Allow disabling default substitutions in the config

Is your feature request related to a problem? Please describe.

When setting things like release to dynamically-generated versions like the ones produced from Git via tools like setuptools-scm, this results in cache invalidation on every single build, effectively rendering it unusable.

Describe the solution you'd like

Projects that don't make use of substitutions like |release| / |version| in their RST files, should be able to disable those so they don't influence the cache and let it be reused.

Describe alternatives you've considered

Setting release and version to static values, but that's not desired.

Additional context

• possibly related discussion: https://github.com/sphinx-doc/sphinx/issues/2173
• definition in the source code: https://github.com/sphinx-doc/sphinx/blob/f85f50e/sphinx/transforms/__init__.py#L37-L42
• https://github.com/pytest-dev/pytest/issues/12538

cc @RonnyPfannschmidt

Start with the default substitution definition in sphinx/transforms/init.py, especially the lines linked in the issue, and trace how release and version affect configuration and cache invalidation. Determine where a configuration option can disable these substitutions, then verify that builds without their use can reuse the cache while existing substitution behavior remains unchanged.