Fix/plugins graph

[fortmarek]

Resolves #4156
Request for comments document (if applies):

Short description 📝

There has been a bug where the Tuist.graph() from ProjectAutomation would not work since TUIST_CONFIG_FORCE_CONFIG_CACHE_DIRECTORY was passed even if it was empty.

There is also a new repo ProjectAutomation which has tuist as a submodule - and users can point to it instead, so they don't have to install all the tuist dependencies which they won't need.

I have also slightly updated the documentation.

Checklist ✅

• The code architecture and patterns are consistent with the rest of the codebase.
• The changes have been tested following the documented guidelines.
• The CHANGELOG.md has been updated to reflect the changes. In case of a breaking change, it's been flagged as such.
• In case the PR introduces changes that affect users, the documentation has been updated.
• In case the PR introduces changes that affect how the cache artifact is generated starting from the TuistGraph.Target, the Constants.cacheVersion has been updated.

[fortmarek]

Thanks @ezraberch for the suggestions!