[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
diff --git a/clang-tools-extra/docs/index.rst b/clang-tools-extra/docs/index.rst
index 2710e40..0906cd6 100644
--- a/clang-tools-extra/docs/index.rst
+++ b/clang-tools-extra/docs/index.rst
@@ -16,7 +16,7 @@
    :maxdepth: 1
 
    clang-modernize
-   clang-tidy
+   clang-tidy/index
    modularize
    pp-trace