Add documentation around custom builds of components

[Nana-EC]

Problem

Currently local node utilizes official builds but it's unclear how a developer might take any one of the sub systems (consensus node, mirror node importer/rest/grpc or relay) and build images that could be used to experiment with early ecosystem changes

Solution

Add a custom-sub-system-builds.md README that shows how to builds each system and load it into docker compose setup for use

• [ ] consensus node (Services changes)
• [ ] consensus node (Platform changes)
• [ ] mirror node (importer changes)
• [ ] mirror node (rest API changes)
• [ ] mirror node (gRPC API changes)
• [ ] mirror node (relay HTTP changes)
• [ ] mirror node (relay WS changes)
• [ ] mirror node explorer

Alternatives

No response