Mostly finalize release documentation

Based on

• https://git.glasklar.is/system-transparency/project/documentation/-/blob/main/archive/2024-04-16-releng-notes.txt?ref_type=heads
• https://git.glasklar.is/system-transparency/core/system-transparency/-/blob/main/keys/README.md?ref_type=heads
• What was already in the repository
• Misc chats

Other than just fleshing things out a bit more and resolving TODOs in the text, I renamed "component release" to "repository release". Because when this is being fleshed out, the definition of collection is essentially repository which feels moot?

Part of #44 (closed).