Consider supplying a more complete example configuration file

This issue has been migrated from [#13306](https://github.com/matrix-org/synapse/issues/13306).

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https://github.com/matrix-org/synapse/pull/12941 replaced the generated 2850-line sample configuration file with a very bare-bones starting point.

I still believe that was a good decision for all the reasons set out in #8159 , however a lot of people have found it surprising so I am opening this issue as a place to discuss whether there is a sensible middle ground.

One thing that might be worthwhile is an example file containing some of the settings you are *likely* to need to configure (eg: url previews, turn server) without attempting to be comprehensive. The CUPS project does something like this (it generates an example `cupsd.conf` from https://github.com/OpenPrinting/cups/blob/master/conf/cupsd.conf.in; the authoritative documentation is managed separately at https://github.com/OpenPrinting/cups/blob/master/man/cupsd.conf.5).