[Docs] Ordering of entries in the "Indexes and Constraints" section in explore

[rkarthik007]

Description

Wanted to fix the ordering to be more “most important & common” to “most advanced” order.

Could we change the order to the following:

• Primary Keys
• Foreign Keys
• Overview --> Secondary Indexes (needs rename)
• Unique Indexes (this needs to have the UNIQUE CONSTRAINT content also, there is no difference between the two in YB)
• Partial Indexes
• Expression Indexes
• Generalized Inverted Indexes --> GIN Indexes
• Other Constraints --> CHECK, NOT NULL Constraints

cc @ymahajan - fyi

[ymahajan]

We can slice to following sections as well.

• Multi-column indexes
• Covering indexes
• Prefix indexes