release-25.2: docgen: update ALTER {INDEX,TABLE} SCATTER diagram

[blathers-crl]

Backport 1/1 commits from #153682 on behalf of @rmloveland.

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

Diagram preview image:

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Release justification:

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes. Includes test-only changes, build system changes, etc.
• Fixes for serious issues. Defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to.

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[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.}

[blathers-crl]

✅ PR #153753 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: This pull request is compliant with the backport policy as it is a non-production code change. According to the policy, changes that affect only non-production code, documentation, tests, or development tools are exempt from the standard backport policy that requires fixes for critical bugs or feature-flag gating. The PR body explicitly states this exemption with 'Release justification: non-production code change'. Additionally, the files affected in this PR include documentation files (docs/generated/sql/bnf/alter_index.bnf and docs/generated/sql/bnf/alter_table.bnf) and a part of the code generation tool (pkg/cmd/docgen/diagrams.go), all of which are classified under non-production changes. There was no need to assess critical bug criteria, feature flagging, or backward compatibility as these are not applicable for this type of change.

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

[cockroach-teamcity]

This change is