[clang-tidy] Move clang-tidy docs to a separate directory. Create doc files for checks

The doc files for checks have been generated from the corresponding header files
using the docs/clang-tidy/tools/dump_check_docs.py script. Committing the script
as well, but the intention is to move all the user-facing docs from header files
to the rST files and add links to .h files appropriately.

llvm-svn: 246173