Consider retiring mkdocs-asf-theme

[jtherrmann]

Jira: https://asfdaac.atlassian.net/browse/TOOL-3478

Note: The above link is accessible only to members of ASF.

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@jhkennedy suggested we might consider retiring this custom theme in favor of the standard https://github.com/squidfunk/mkdocs-material theme. I would probably agree, since keeping this theme up-to-date with upstream changes is a significant amount of work that involves copy-pasting (for example, see #44).

If we decide to move forward with this, we'd probably want to:

• Try switching https://github.com/ASFHyP3/hyp3-docs to the upstream theme to see how severely it would impact the look and feel
• Discuss with any other ASF teams who may be using this theme and offer to hand off ownership of this repository

[jhkennedy]

We'll also want to make sure we:

• Keep the unified search bar (using standard mkdocs-material overrides)
• Connect the site to Google Analytics appropriately