[clangd] Update features table in the docs with links to LSP extension proposals

Also update the semantic coloring entry to reflect it being supported in
clangd now.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65373

llvm-svn: 369229