Support `start-after` option for `.. include::` directive

[Frank Harkins]

Problem Description

I want to include my README in the index of my API docs, but this results in a duplicate top-level heading as the README has its own title. It's quite a specific use case but probably one that's likely to recur.

Screenshot

Proposal

Implement the start-line option for the include directive. Then the following code would resolve the problem.

.. include:: ../README.md
   :start-line: 1

Alternatives

Users can use CSS to hide the README heading by its id, but the navbar still includes the extra heading.

Additional context

I'm happy to make/review the PR if you think this is a good idea. For completeness, I can include the other start-* and end-* options but I think the other options are out of scope.

[Maximilian Hils]

Happy to take a look at a PR! :)

I don't think we have anything else yet that parses directive options, but if we tackle this it probably makes most sense to extract it into a generic helper like so:

def _rst_extract_option(contents: str, option: str) -> tuple[str, str | None]
    """
    Extract the specified directive option from contents.
    Return updated contents with the option removed and the value of the option if present.
    """

I haven't looked too closely though, feel free to suggest something smarter. :)