Add support for :glob: option to the feed directive

[bosonogi]

It mimics the way :glob: works for the toctree directive.

[doismellburning]

Thank you! But I think this might have a bug around how paths are constructed.

The following in source/notebook/index.rst (adapted from a very very similar toctree directive where * just worked):

.. feed::
    :title: Notebook
    :rss: notebook.rss
    :glob:

    *

where e.g. I'm looking to include source/notebook/2025-04-28-whats-a-backlog-anyway.rst, gives me Sphinx build errors of the form:

/Users/me/git/bliki/source/notebook/index.rst:20: WARNING: feed contains a reference to nonexisting document 'notebook/notebook/2025-04-28-whats-a-backlog-anyway' [docutils]

(Note the double notebook/notebook)

I can't yet work out if you're accidentally adding something twice or not dropping something though...

[bosonogi]

Somehow this did not affect my file layout, but I see the issue now. Documents are globed from a list of previously collected documents, and so those names should not be modified. Only the names that are not glob patterns need to be joined with env.docname.

[doismellburning]

@bosonogi That does the job nicely, thanks!

You might like to also grab doismellburning@840e981 which adds support for :reversed:!