Add documentation for HYPO indexes

[mjonss]

Change Request

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.
   There are no documentation of the HYPO index type, implemented here.

2. Describe your suggestion or addition.
   A Hypothetical index type was added in X.Y

with syntax like:
ALTER TABLE t SET HYPO TIFLASH REPLICA ...
DROP HYPO INDEX ...
CREATE INDEX <index_name> TYPE HYPO ON <tbl> (<columns>)
...

This is to make it possible to evaluate how the optimizer would take a new index into account, if it would be used, what plans would be updated if such index existed etc.

As I understand it, it will only affect EXPLAIN commands, and not any other statements, since it is just a hypothetical index, metadata only, no actual index on any data.

3. Provide some reference materials (such as documents and websites) if you could.
   planner: support hypo-indexes tidb#43817

[dveeden]

Also: recommend index show etc from pingcap/tidb#55947