Switch documentation to mdBook and add missing pages

[turt2live]

• What is policyserv
• How to run the thing (and why the setup bot exists)
• Docker
• Grafana Metrics
• What to look for in terms of alerts/problems
• How to use the Foundation's instance
• How to configure your community's filters via setup-bot
• How to use the server-centric community API
• How to use the administrative API
• How to set up Search Redirection in Synapse (and other vendors, if available)
• How to set up HMA