[clangd] Update clangd-vscode docs to be more user-focused.

Summary: Relegate "updating the extension" docs to a separate file.

Reviewers: hokein, kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67092

llvm-svn: 371390