Add OpenGraph metadata to docs via Sphinx extension

[justinmayer]

Uses the sphinxext-opengraph Sphinx extension to generate OpenGraph metadata for Pelican's documentation.

[pauloxnet]

Uses the sphinxext-opengraph Sphinx extension to generate OpenGraph metadata for Pelican's documentation.

+1 for me

Do we need to add some variables in the config?
https://github.com/wpilibsuite/sphinxext-opengraph#example-config

[justinmayer]

From the Options section of that same README:

Users hosting documentation on Read The Docs do not need to set any of the following unless custom configuration is wanted. The extension will automatically retrieve your site URL.

[pauloxnet]

LGTM

[avaris]

👍🏻

[justinmayer]

@pauloxnet / @avaris: Many thanks for taking a look at this and providing feedback 😁