Explain the different approaches to onionify a website

The overview should be easy to understand for TB users and website administrators. The idea is to make this a section in https://www.torproject.org/docs/tor-onion-service.html.en

• Proposal: https://gitweb.torproject.org/tor-browser-spec.git/tree/proposals/100-onion-location-header.txt
• Summary: https://medium.com/@alecmuffett/different-ways-to-add-tor-onion-addresses-to-your-website-39106e2506f9
• Discussion: https://lists.torproject.org/pipermail/tor-onions/2018-September/000301.html
• Addon option: legacy/trac#19812 (moved)
• test: https://perfectoid.space/test.php (cloudflare)

With legacy/trac#24553 (moved) TB 8 honors alt-svc headers and silently uses the onion service (legacy/trac#27502 (moved)) but does not show it in the address bar or circuit view (legacy/trac#27590 (moved)).

For reference see legacy/trac#21952 (moved).