[Docs][Format] Add how to change format specification

[kou]

Describe the enhancement requested

We should have a document how to change our format specification under https://arrow.apache.org/docs/format/ .

It must be based on our discussion: "[DISCUSS] Format changes: process and requirements" https://lists.apache.org/thread/9t0pglrvxjhrt4r4xcsc1zmgmbtr8pxj

Component(s)

Documentation, Format