:book: Refresh documentation and improve "Concepts" structure

[embik]

Summary

I sat down and reviewed some of our documentation, as kind of a post-0.25 release clean-up. Work is split across several commits to give reviews an idea of the changes, but the big picture is:

• I did a re-org of mostly the "Concepts" chapter. I felt that content wasn't grouped the optimal way and would make more sense this way, including future extensibility of sub-chapters. Here's a before/after view:

Before	After

• I tried to apply title case more consistently. Some pages used it, some didn't, so I tried to unify page titles.
• The project goals page was still mostly what it was a couple of years (pre-community-governance) ago. I've removed the timeline (because that time has passed since the document was written) and the references to the goals we put on hold by moving TMC out of core kcp.
• I started to take care of the "Exporting APIs" page. IMHO it's an extremely important page because it's the "walkthrough" for API providers, but it had a lot of TODOs and was using the old API domain. I refreshed existing parts of this page and started documentation for controller implementation, including e.g. endpoint slices. The APIBinding part of this page is something I'd like to work on in a follow-up.
• A few minor tweaks like showing a repository name and a "back to top" button.

Related issue(s)

Fixes #

Release Notes

NONE

[kcp-ci-bot]

LGTM label has been added.

Git tree hash: 9e7959ef121972f4ac0ecd36438d40d6eff97194

[kcp-ci-bot]

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