This builder builds CMake using lint tools but does not run the test
suite.  Document it for reviewers to request with `Do: test -i lint`.

Move the C++ subset rules from the `cmake-developer(7)` into our
dedicated coding guide since they do not need to be in user-facing
documentation.

Simplify the `CONTRIBUTING.rst` document by moving the code style
section over to our dedicated coding guide and linking to it instead.

Create a dedicated document to cover the CMake source code for
developers.

Add a dedicated subsection header for its documentation.

Fixes: #16264.

Teach Sphinx and our own install rules to skip a `Help/dev` directory.
This will give us a place to put developer-only documentation that
should not be included in the user-facing documentation.  Add a
placeholder README.