docs: add mermaid diagram support

[choejwoo]

Closes #26108

Note: If this is the only Mermaid diagram we expect to have (and there’s no plan to add more), we could consider skipping Mermaid support entirely and using a static image instead. I’m happy to switch to that approach if preferred.

I added Mermaid support to the docs.

• I avoided loading Mermaid from a CDN since it runs at runtime in the user’s browser and can be blocked (CSP/air‑gapped/corporate networks). For stability, I vendored Mermaid latest release v11.12.2 into the repo and included the upstream license and attribution files. There is a trade-off (repo size), but the file size felt acceptable(~3MB). If you prefer using a CDN or have a better approach, please let me know.
• Added Mermaid initialization so diagrams are rendered properly (including with MkDocs Material navigation).
• Mermaid blocks aren’t reliably identifiable in the rendered HTML (the language class isn’t preserved), so I detect Mermaid diagrams by matching known Mermaid “block starter” keywords (e.g., sequenceDiagram, graph, flowchart) in code blocks.
• Verified it renders correctly in my environment on Chrome and Safari.
• Improved the Mermaid diagram syntax/readability.

Below are screenshots from my local tests on Chrome.
Thanks!

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Title of the PR
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

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