Improve sysadmin documentation website

• General goal: onboarding material + transparency about how we run the archive.

• Target user: team member, partners (e.g. mirror operators), or contributor who needs a clear view of the infrastructure architecture.

This task will be completed when it:

• Documents the configuration system of each component.

• Documents hardware architecture.

• Documents CI architecture (and other major services currently not documented).

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Migrated from T4082 (view on Phabricator)