Moved clangd docs to a separate directory in preparation to restructure them into multiple files

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58607

llvm-svn: 354786