[Docs] Intro page for indexes and constraints in explore section

[rkarthik007]

Description

We need some introduction on the landing page itself, just like we do for Transactions. This should cover the following:

• A high level paragraph talking about the following:
  • How the reuse of PG means we support a lot of indexes and their semantics
  • How indexes use distributed transactions internally.
  • We should eventually link out to a docs page in the develop section once its ready
• Have a table about which index types are supported in YSQL and YCQL - use this table to add links to the various sections.
• Talk about differences from PG: BTREE indexes, etc
• Talk about what PostgreSQL indexes are not supported, and link to the page that shows what PG functionality is not supported. Example: GIST, etc (with a link to the github issues)