Theme styling for Prefect brand

[tpdorsey]

Summary

Makes some changes so collection docs more closely align with Prefect documentation appearance.

Updates configuration and CSS:

• color and font match Prefect 2.0 docs
• prefect icon and favicon
• github icon, link to repo, link to edit
• table formatting for readability

Some open questions regarding collections docs:

• should style and styling be aligned with Prefect 2.0 docs? (I think so)
• should the styling be applied to all collections as part of the boilerplate for all new collections?

Relevant Issue(s)

Checklist

• [ ] Summarized PR's changes in CHANGELOG.md

[ahuang11]

Thanks for making these changes! Maybe once this is reviewed, we can port it over to the template as well! https://github.com/PrefectHQ/prefect-collection-template

[billpalombi]

Regarding the open questions:

• I agree with you that the style should be aligned with Prefect 2.0 docs.
• Yes, I think the styling be applied to all collections as part of the boilerplate for all new collections.