schemachanger: add doc.go

[postamar]

Fixes #104779.

Release note: None

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[postamar]

cc @rafiss @Xiang-Gu @chengxiong-ruan @fqazi for some proofreading please

[chengxiong-ruan]

looks very good to me. thanks for writing this. I personally probably know most of the stuff, but never be able to describe them systematically like this.

[Xiang-Gu]

1. somewhere in the doc, preferably in a notable place, we should explain why it's called "declarative" schema changer.

2. I think all the efforts we made into testing this framework also deserves a section. In my opinion, it's quite sophisticated and showcases the testability of the framework -- end-to-end, rollback, puase/resume, backup/restore, DML injection, three explains, and mixed version tests.

[postamar]

Those are all excellent comments, thank you very much for having given this a proper read-through!

[postamar]

I've applied almost all of the suggestions, thanks again!

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)