docgen: make automated docs generation more efficient

[RichardJCai]

See:
https://github.com/cockroachdb/cockroach/blob/v21.1.1/build/teamcity-diagram-generation.sh

Right now we generate both the bnfs/svgs in the job no matter what. This takes more CPU time than needed.

We can be smarter about this by

1. Only generate the bnfs if sql.y or diagrams.go has changed.
2. Only generate svgs if the bnfs have changed (this is easier, bnf generation is quick whereas svg generation takes the bulk of the time)

2 is likely easier - we can also only generate SVGs for bnfs that have changed.

Note that this has to be backported to 20.2 and 21.1 as well if a change is made.

Side note: the TC job fails right now if the git push does nothing, (no changes) we should fix this.

Jira issue: CRDB-7832