Document that when using RBE, users should ensure the locally installed Xcode toolchain version matches that of the bots, otherwise they're likely to experience slow builds due to cache misses.
Ideally, we should also check the value automatically against the `sdk_version` specified in the relevant `et` build config itself.
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
This PR changes from using a different GN pool for each non-compiler tool, to using one GN pool for everything.
Additionally, since we are no longer linking remotely in any configuration, this PR uses the pool for linking.
ObjC and impellerc tasks also do not (yet) run on RBE, so they are also now governed by the pool.
Needs https://github.com/flutter/buildroot/pull/856
## Description
This turns off the Doxyfile option `CREATE_SUBDIRS` because it prevents deep link URLs from being stable. It means that thousands of files will be in the main directory, but the filesystem should be able to handle that.
I got rid of the header/footer files for doxygen (because Doxygen will generate them anyhow) and fixed where it was looking for the Flutter logo.
I also ran Doxygen on the Doxyfile template and Doxyfile and updated all the options to correspond with the latest Doxygen.
