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

[mhils]

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. :)