DOC: add section on Interval

[jreback]

this could be top-level (and include indexing with IntervalIndex). Mainly showing how to work with Interval.

• add Interval to API.rst

[alexlenail]

@jreback I could potentially take this on with some guidance from yourself and @shoyer. However, I would probably want to document the "new behavior" I'm advocating for in #16316 (and defining in #16386). I'm curious -- what would be the release timeline for new Interval / IntervalIndex behavior? Would that constitute a minor release (20.2) or a major one (21.0) ?

[jreback]

0.21.0, this is changing lots of things. minor releases are for bug fixes (and minor ones at that).

[starlord1311]

@jreback i would like to take up this issue. can i?

[jreback]

sure!

[starlord1311]

can you help me with what i am supposed to do? @jreback