docs/generated: stop checking-in files

[irfansharif]

Is your feature request related to a problem? Please describe.

We currently check in generated doc files. A few of them see very frequent change (https://github.com/cockroachdb/cockroach/blob/master/docs/generated/settings/settings-for-tenants.txt) for e.g. and a frequent source of benign merge conflicts and forgot-to-generate-and-need-to-rerun-CI activity, which is not great and time consuming.

Describe the solution you'd like

Do we get much out of these things being checked in? If not, we should .gitignore them.

Jira issue: CRDB-13319

[irfansharif]

@knz, do you know off-hand if we need to be checking in these files?

[github-actions]

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