Move sloth file generation over from autometrics-rs repo

[emschwartz]

https://github.com/autometrics-dev/autometrics-rs/tree/main/autometrics-cli

And update docs to point to new location:

• https://github.com/autometrics-dev/autometrics-shared#prometheus-recording--alerting-rules
• https://github.com/autometrics-dev/autometrics-rs/blob/f5d4040c7fbaaecda51a4eff195b27cba6956b58/autometrics/src/objectives.rs#L139
• https://github.com/autometrics-dev/autometrics-rs/blob/f5d4040c7fbaaecda51a4eff195b27cba6956b58/autometrics/src/README.md?plain=1#L130