Turn TRaSH metadata rendering into a generic rendering step

[Callum027]

This would be a good way to clean up handling rendering of instance links and substituting resource IDs for names before update_remote is run.

1. Move the TRaSH-Guides metadata folder into global state, rather than passing it as a parameter
2. Add the following manager method, which will call into corresponding Config methods by default. The point of passing both the local and remote instance configs at the same time is to allow one set of requests to render both objects at once.

   class Manager(Generic[Config, Secrets]):
       def render_local_remote(self, secrets: Secrets, local_instance_config: Config, remote_instance_config: Config) -> Tuple[Config, Config]:
           return type(local_instance_config).render_local_remote(local_instance_config, remote_instance_config)

   • Or should this be a per-Config object render method? the corresponding requests would have to run twice, but it would be much simpler to implement.