docgen: recognize "unimplemented" as marker to ignore rules

Various aspects of our SQL grammar are marked as "unimplemented". These unimplemented features/keywords/etc. should not be included in the generated SQL diagrams we use in our docs. Currently, we must edit diagrams.go to hide unimplemented syntax from the generated bnfs that are used to genereate our SQL diagrams. Instead, it would be great if "unimplemented" were treated like /* SKIP DOC */. This would be much more efficient in that all unimplemented features would be excluded from all generated bnfs without docs writers needed to tweak diagrams.go.

Example: https://github.com/cockroachdb/cockroach/pull/45442

Jira issue: CRDB-5153