Add documentation for custom sync server preference screen

[oakkitten]

As promised in ankidroid/Anki-Android#12367, this is my stab at the documentation for the custom sync server preference screen, to be added when the documentation is adjusted to reflect the new preference structure.

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Custom sync server

As an alternative to using AnkiWeb, you can use your own server for synchronization between devices. You can host it on your website, or you can run it on your laptop and synchronize AnkiDroid with it, even if you are not connected to the Internet.

There are two server applications that you can use. One is built into Anki itself; it is easy to use, but it lacks security and at the moment cannot synchronize media files. Another one, Ankisyncd, supports multiple users and media synchronization, but must be installed separately.

A server can have either one HTTP endpoint for synchronizing the collection, or two for synchronizing the collection and media files, respectively. It is possible to use two different servers; Anki documentation suggests using AnkiWeb for synchronizing media and the built-in server for synchronizing collection. However, you cannot log in into them separately.

To use a custom sync server, please follow the documentation of the server of your choice to install it, then enter the full endpoint URL(s) (e.g., http://example.com:27701/sync/) in the Custom sync server preference screen. You may use both https:// and http:// URLs; mind that using http:// is not safe outside of your local network. If you don't specify an URL, the according default AnkiWeb server will be used instead. After you set up the servers, you can log in and synchronize like you normally would. If you are using Anki desktop, you will also have to set it up to use the custom server.

If you don't have Internet, you can create a local network between your laptop and phone by using tethering initiated by either device. It might be more convenient to connect your phone to your laptop, not the other way, as in the latter case the laptop may get assigned a different IP each time you turn on tethering. You can use these instructions for Windows 10. If the connection doesn't get through, you may have to adjust your firewall settings.