doc: add sphinx extension improving only directive

The Sphinx ``.. only::`` directive is limited to handling only
conditional text and can't handling conditional use of directives
For example,

```
.. only:: test

   .. automodule:: west.runners.core
      :members:
```

is not handled. This PR monkey patches the handling of the existing
``.. only::`` directive done by Sphinx.

See https://github.com/pfalcon/sphinx_selective_exclude

 for details.
Licensing amended to Apache 2.0 with permission from the author.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>