[webgui,net,graf] move builtins to subdir for consistency

[ferdymercury]

Spinoff of #21904 to easen review

[github-actions]

Test Results

    22 files      22 suites   3d 11h 45m 17s ⏱️
 3 864 tests  3 864 ✅ 0 💤 0 ❌
77 139 runs  77 139 ✅ 0 💤 0 ❌

Results for commit 66b6820.

♻️ This comment has been updated with latest results.

[dpiparo]

Thanks for this work. While @linev and @bellenot have a look, let me perhaps note that it could be that at the moment we do not plan to have render core or mathjax on lcgweb by policy. This is due to the fact that we think to lcgweb as a place where the "easy" external packages can be added as backup, where "easy" means installable via the package managers of the linux distributions for which binaries are provided and Homebrew (which we consider the "macOS package manager").
Said that, thanks for these changes, they improve ROOT and provide clarity to its users and developers, kudos

[linev]

Thanks for taking care of it.