[clang-tools-extra] [clang-doc] fix CSS, JS paths for HTML Mustache generation (PR #160360)

[Erick Velez via cfe-commits]

evelez7 wrote:

This fix has to delete the HTML Mustache unit tests because I don't think it's possible to create nested temporary directories like what the unit test was doing. We'd need a temp "HTML" dir inside the temp "createResourcesTest" dir. I'm not sure if this also aligns with the previous "anti-pattern" of creating/checking files during unit tests.

I could avoid deleting the unit test if the resource files were placed one level above the `HTML` dir (so in the context `RootDir`), but then the templates would need to refer to `../clang-doc.css`, etc. That would mean the `HTML` dir cant be moved around easily.

https://github.com/llvm/llvm-project/pull/160360