[Documentation] mention the existence of the RST `.. code::` directive about sphinx HOT 1 OPEN

There is a historical reason why we have these two directives but long-story short, .. code is a docutils directive (that we support as a pure docutils) and .. code-block is a Sphinx one (which is an improvement of the latter).

We don't really want to mention the two of them because what is generally needed is the code-block directive and not the code directive. If we were to mention it, users would maybe try to use the ..code directive.

There was a very old issue (#2155) that I recently closed where it was debated whether to keep code synced with the docutils one or not. One maintainer personnally preferred merging these two together but eventually made a PR where code and code-block are distinct. While I also share this opinion, having distinct code directives might be useful for extensions that assume that the code directive is compatible with docutils and has minimal processing in their run method, so I would not change the behaviour.

Nevertheless, what we can do is mention the code directive but stress that code-block is better for most usage.

from sphinx.