Rendered docs do not show default pydantic field values

I've just noticed that e.g. the server config docs do not show the default values for fields, e.g.,

I think it would be very helpful if they did; this may require an expansion of our existing manual configuration docs at https://www.optimade.org/optimade-python-tools/configuration/.

We should also consider removing internal classes like the pydantic Config of our ServerConfig model as it adds a lot of clutter (and this case, has a very confusing name...).

A related issue is the use of enums in our config, the allowed values of e.g. database_backend are not at all clear unless you scroll all the way down to the bottom of the page to the docs for the SupportedBackend type itself. There must be a better way of doing this...

Start with the rendered configuration documentation at optimade.org/optimade-python-tools/configuration/ and compare it with the ServerConfig and SupportedBackend material described in the issue. Trace how those models are exposed in the rendered docs, then define completion as showing field defaults, reducing internal Config clutter, and making allowed database_backend values clear.