Exclude dependency docs from source tarball

[cameel]

Follow-up to #16256.

During the prerelease we discovered that create_source_tarball.sh packages nlohmann-json docs as a part of our source. And it includes a bunch of unnecessary things, especially big images and rendered versions of the docs. For example:

• https://github.com/nlohmann/json/blob/v3.11.3/docs/avatars.png (1.6 MB)
• https://github.com/nlohmann/json/blob/v3.11.3/docs/json.gif (1.6 MB)
• https://github.com/nlohmann/json/blob/v3.11.3/docs/usages/macos.png (1.2 MB)
• https://github.com/ericniebler/range-v3/tree/0.12.0/doc (gh-pages/ is 15 MB)
  • I see that it actually does not end up in the archive. I think we skip recursive submodules but it's probably not even intentional. If it was a fixed part of range-v3 we'd have it in there as well.

Excluding doc/ and docs/ from these submodules shaves off almost half of the size of the source tarball (12.3 MB -> 7.3 MB).

These things are essentially dead weight as I would not expect anyone to look for them in our archive (they'd more likely go the source repo or to their docs webpage) and they do not the affect compiler build.

[cameel]

I was about to merge this, but decided to take one last look at c_source_tarball. For some reason it still produces a tarball that includes the docs, even though locally the docs were being excluded back when I created and tested the PR. Will have to investigate. For now reverting back to draft.