Update client, server and map feature pages and screenshots

[ChillerDragon]

https://ddnet.org/client/

For example preview of /spec tees

[Robyt3]

The pages for server and map features also need an overhaul.

[Robyt3]

I would also consider moving the pages to a common parent, i.e. /features/client/, /features/server/, /features/map/.