docs: reorganize executor administration and deployment guidance

[DaedalusG]

Closes PLAT-790

Summary

This reorganizes the executor documentation around a clearer path from administration concepts to infrastructure deployment and ongoing operations.

• Refocuses admin/executors on executor concepts, Sourcegraph instance setup, authentication, monitoring, secrets, and sandboxing.
• Turns self-hosted/executors into the deployment entry point, with a concise prerequisite checklist, deployment decision guidance, validation, and links to each supported deployment method.
• Moves deployment-specific architecture, procedures, configuration, upgrade, and security guidance into the relevant Binary, Docker Compose, Kubernetes, Firecracker, AWS, and Google Cloud pages.
• Splits private registry configuration, custom certificate configuration, and executor capacity planning into focused pages that are linked from the relevant deployment guides.
• Removes the ambiguous deploy-executors catch-all page, preserves its useful content in the new structure, and adds redirects for its former URLs.
• Improves sidebar navigation, in-page indexes, cross-links between administration and deployment content, and links from feature documentation to the appropriate executor guidance.

Testing

• Tested the updated documentation and navigation locally with npm run dev.

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Project	Deployment	Actions	Updated (UTC)
sourcegraph-docs	Ready	Preview, Comment	Jul 22, 2026 11:04pm

[DaedalusG]

Encorperated changes from #1820