[clangd] Update documentation page with new features, instructions

Summary:
- Some features were implemented so mark them as such.
- Add installation instructions (LLVM debian packages)

Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>

Reviewers: jkorous-apple

Reviewed By: jkorous-apple

Subscribers: jkorous-apple, klimek, ilya-biryukov, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D41306

llvm-svn: 320988