Release branches

[fizzyizzy05]

Some plans for release branches

• We should probably have stable ahead of the alpha release, so users installing for the alpha can be smoothly transitioned as the project progresses. This way we can make the churn from latest opt-in, rather than opt-out.
• stable should probably build nightly or something at first, just so we can ship out bug fixes and features sooner. this won't build on push, so there's more space to revert/change things before it gets to users. When deemed appropriate, the stable stream should probably slow down to around once a week, with additional builds where appropriate.

apollo-linux/docs should also be updated as appropriate.