Remove obsolete docs links.
Remove downloading of the old releases list, and bundle the file in the repo.
Also the message in update-tags was wrong: we need git to do anything there now, as update-tags gets emscripten-releases using git currently (which is ok as update-tags is run by users that installed with git; otherwise they installed without git and just update the entire emsdk normally).
emsdk install tot-upstream will install the very latest build from emscripten-releases. These builds are useful for emscripten github CI.
There isn't a latest or lkgr for emscripten-releases currently. What this does instead is get the git repo, and check if builds exist for the latest commits there, returning the latest of those.
This also makes us not update the emscripten-version.txt file if we are not an actual version. That is, for a tot build we leave that file unchanged in the emscripten checkout.
There is also a tot-fastcomp for fastcomp.
