Fix outdated repo docs

[wileyj]

This PR attempts to update all docs relating to the release process for both stacks-core and stacks-signer. Additionally, some cleanup was performed to remove files that are no longer needed (or won't be needed post 3.0) - specifically, some configs for network modes that won't be present were removed.

release process files were updated to reflect the current gitflow-ish branching strategy, with a dedicated document ./docs/branching.md defining how branches should be named.