Docs: Federation troubleshooting re: Cloudflare

This issue has been migrated from [#16228](https://github.com/matrix-org/synapse/issues/16228).

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**Description:**

I am using Cloudflare as my DNS provider. When attempting to federate (using port 8448), I was getting errors like

```
Get "https://104.21.90.241:8448/_matrix/key/v2/server": context deadline exceeded (Client.Timeout exceeded while awaiting headers)

"https://[2606:4700:3036::ac43:a2cf]:8448/_matrix/key/v2/server": dial tcp [2606:4700:3036::ac43:a2cf]:8448: i/o timeout (Client.Timeout exceeded while awaiting headers)
```

I determined this to be caused by the fact that, [when using Cloudflare's proxying only certain ports are allowed](https://developers.cloudflare.com/fundamentals/reference/network-ports/). The simple solution is to turn off proxying (set matrix DNS entry to "DNS-only").

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To elaborate, what happens (in my case) is:

- `https://example.com/.well-known/_matrix/server` is fetched, it delegates to `https://matrix.example.com`
- `https://matrix.example.com` resolves to a Cloudflare proxy IP address
- Attempt to reach `https://:8448`
- Cloudflare unhappy :(

You could presumably also change your configuration to use one of the supported ports.

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I think a note in the delegation and/or federation docs about usage with Cloudflare would go a long way here to save some headache.