📖 Update documentation dependencies and add dark mode

[embik]

Summary

This PR is a general sweep at our documentation and does the following things:

• Update all python dependencies used to build the documentation. I've eliminated "saved" dependencies of the packages we actually need, so we don't need to go over updating all of them. We only care about mkdocs, its extensions and mike.
• Add a dark mode based on device preferences, but it can also be switched via the navigation toggle.
• Rename the page from "kcp Documentation" to "docs.kcp.io" because I much better liked the look of it in the top navigation.
• Make the top navigation match the theme by either turning white or black for good contrast with the kcp logo.
• Transform top-level sections into tabs. I have not reordered content, but in my opinion this is the perfect way to group documentation by personas (users, admins, developers) in the future.
• Update logo and favicon to the new official artwork.
• Remove the OWNERS file so the top-level OWNERS file is effective in docs/ in the future.

The new docs website can be reviewed by running make deploy-docs serve-docs on this branch, but here's two screenshots to get a feeling:

Light Mode	Dark Mode

Related issue(s)

Release Notes

NONE

[embik]

/cc @sttts

[sttts]

/lgtm
/approve

[kcp-ci-bot]

LGTM label has been added.

Git tree hash: a4493dee8afb47ef5625ba4f27af50c8ebd79bda

[kcp-ci-bot]

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