Add interactive command graph to docs

[Markus Herdieckerhoff (markusherdi)]

Summary

• Adds docs/command-graph.html, an interactive, searchable visualization of every content-cli command (grouped by area, with descriptions/options shown on click).
• Links to it from the README's new "Command Graph" section via raw.githack.com, so it renders live in the browser without needing to be downloaded.
• Documents in docs/how-to-add-command.md that the graph must be updated manually whenever commands are added, removed, renamed, or changed.
• Removes the stale Agentic Development link (docs/user-guide/agentic-development-guide.md) from mkdocs.yaml and docs/user-guide/index.md — the file was deleted in SP-880: remove asset-registry-endpoints agent skill #380 but the links were never cleaned up.

Test plan

• Merge and confirm https://raw.githack.com/celonis/content-cli/main/docs/command-graph.html renders the interactive graph
• Click through a few nodes in the graph to confirm descriptions/options show correctly in the sidebar

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[Jing Sun (kuvia)]

While I think the graph is very useful I'm not too keen on required manual updates whenever we change something (which is pretty much every PR). Since you generated it, can it be genereated? It takes a ridiculous amount of discipline to make sure such a thing doesn't drift.