release-23.1: docgen: update BEGIN and SET TRANSACTION diagrams without iso level

[taroface]

Backport 1/1 commits from #137368.

/cc @cockroachdb/release

---

Updated the BEGIN diagram for v23.1, which had been empty since this commit. (#137058 covers v23.2 and later.)

Also updated the SET TRANSACTION diagram, which had not been displaying most of its syntax.

Epic: none
Release note: none
Release justification: non-production code change

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
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• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[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 

[yuzefovich]

LGTM, but curious about the need for backports to 23.1 and 22.2 branches - I think we might not have any more scheduled releases on them, what's the reason for backporting this change? Are the docs pages for the corresponding release somehow influenced by this?

Reviewed 10 of 10 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @taroface)

[taroface]

TFTR! Yes, this is strictly to fix some broken diagrams (technically generating new HTML files in generated-diagrams repo) so that they will appear properly on the docs pages for the corresponding version. The BEGIN diagram being broken since 22.2 was identified as an issue.