Start UCS docs with examples and use cases

Users interested in UCS are primarily going to be interested in examples of how UCS should be used for certain types of workloads.  We start the current docs by saying it can replace every other compaction strategy, but leave it up to the user to figure out exactly what that means for them.  

Before the docs that explain how it works, let's describe how it should be used.  Users interested in the nuts and bolts can scroll down to learn the details, but that shouldn't be a requirement to switch from an existing compaction strategy to UCS.

A table showing examples of LCS, STCS, and TWCS converted to UCS would suffice for 99% of people's needs.  

More information in this Slack thread: https://the-asf.slack.com/archives/CK23JSY2K/p1707700814330359