Add support for meta description

[hugovk]

Fixes https://github.com/wpilibsuite/sphinxext-opengraph/issues/65.

Re: https://github.com/wpilibsuite/sphinxext-opengraph/issues/65#issuecomment-1285497461.

When ogp_enable_meta_description=True, only add a generated <meta name="description" content="..."> tag if the page doesn't already have a manual one.

For example, a manual override may have been added using this RST:

.. meta::
   :description: My manual description

---

Also fix a bug where it was adding multiple meta descriptions for subpages.

TODO:

• [x] Rename config option to ogp_enable_meta_description
• [x] Document new option
• [x] Address https://github.com/wpilibsuite/sphinxext-opengraph/issues/65#issuecomment-1286542474

[TheTripleV]

Looks good so far. Please see the comments on #65 and #71

[hugovk]

Rebased to resolve conflicts, and renamed option to ogp_enable_meta_description. Still need to address https://github.com/wpilibsuite/sphinxext-opengraph/issues/65#issuecomment-1286542474 and document.

[hugovk]

Updated for https://github.com/wpilibsuite/sphinxext-opengraph/issues/65#issuecomment-1286542474.

Still TODO: add documentation.

[hugovk]

Docs added, ready for review!