docs: add documentation for checkpatch

Add documentation for kernel script checkpatch.pl.
This documentation is also parsed by checkpatch to
enable a verbose mode.

The checkpatch message types are grouped by usage. Under
each group the types are described briefly. 34 of such
types are documented.
Signed-off-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
Link: https://lore.kernel.org/r/20210226093827.12700-2-dwaipayanray1@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/dev-tools/index.rst

@@ -14,6 +14,7 @@ whole; patches welcome!
 .. toctree::
    :maxdepth: 2
 
+   checkpatch
    coccinelle
    sparse
    kcov