Create docs for the project

Needs to cover:

 * How to create a server
 * How to customise the repository that is used in the server (e.g. DI via https://github.com/simple-repository/simple-repository-server/pull/14/files#r2003800882 and https://github.com/simple-repository/simple-repository-server/pull/14/files#r2003802009)
