CEP-19: Add trie memtable docs-storage engine, create and alter table

[polandll]

Authored by Lorina Poland (polandll); Reviewed by ?? for CASSANDRA-18236

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The Cassandra Jira

[smiklosovic]

@polandll I see that @blambov is already reviewing this so I will focus on other doc PR you have created recently.

[polandll]

Still waiting on another review. Merges to trunk and 5.0 when approved.

[blambov]

Two of the comments above (the ones not marked "Outdated") are marked as resolved but have not been.