[clangd] Add documentation page

Summary:
Add a simple documentation page for Clangd.
This will be useful for interested users and contributors to get basic information about how
to get started and the progress of Clangd.

Reviewers: krasimir, bkramer

Reviewed By: krasimir

Subscribers: Prazek, jbcoe, JDevlieghere, mgehre, JonasToth, kromanenkov, xazax.hun, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D31887

llvm-svn: 302191