Sphinxext-opengraph

Fixed a major issue preventing the extension working on normal Sphinx installs

- Implement RTD Support (30)
- Fix bug with ogp_image_alt default (29)
- Use first image (after refactor) (26)
- Refactor codebase (24)

Full list is available [here](https://github.com/wpilibsuite/sphinxext-opengraph/commits/master)

- Exclude raw nodes from description (23)
- Add black formatter (17)
- EOL EOF (16)
- Add support for og:image:alt (15)

- Code refactors
- Add support for custom tags

ogp_custom_meta_tags = [
'<meta property="og:ignore_canonical" content="true" />',
]

- Numerous refactors
- Fix bug if description length was less than 3
- Use the first header in the page as title
- Skip admonitions and invisible content for page description