CASSANDRA-18232: CEP-26 Unified Compaction Strategy docs

[polandll]

New UCS docs.

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The Cassandra Jira

[polandll]

I have sent Branimir Lambov a section of this document for technical review - the example in Basic Sharding, and the following Full sharding discussion. I am waiting for that review.

[polandll]

Closing in lieu of new PR with no merge conflicts.