Modularity improvements

[ThetaSinner]

• Enable Holochain feature overrides #34
• Add holo-dev-server? Could be an extra template, or can just be added by scaffolding #27
• Add documentation #32
  • Default usage, how to set up a flake
  • How to choose your versions (by branch)
  • How to redistribute Holonix
  • Use your own Rust by not setting the rust package input
• An equivalent of the hc-introspect command #25
• Keep the existing shell hook (somewhere?), but maybe just the prompt and not the echo

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• Make it more modular by breaking up holochain into the individual binaries so people can really choose just what they want #44
• Test whether the mold linker gives a significant performance gain and provide it on Linux if so?
• Automatic Rust version updates #98
• Update other flake inputs in CI automatically too. Maybe at the same time as the Rust updates?