Document and recommend default_server_name

[jryans]

This the recommended way to define a default server (instead of the existing HS
and IS URL options). It will use .well-known discovery to find the URLs.

See #8763.

[alexte]

neither config.sample.json nor https://riot.im/develop/config.json has default_server_name.
If this is the new default it should be at least in config.sample.json

[jryans]

neither config.sample.json nor https://riot.im/develop/config.json has default_server_name.
If this is the new default it should be at least in config.sample.json

We're planning some future improvements in this area that may continue to morph the config structure. For config.sample.json, we'll probably rely on a script like this going forward.