Document how to remove git-crypt from a repo

I'm having an issue where git-crypt is failing to unlock in my CI/CD pipeline.

```
git-crypt: error: encrypted file has been tampered with!
error: external filter "git-crypt" smudge failed 1
error: external filter "git-crypt" smudge failed
fatal: functions/.runtimeconfig.json: smudge filter git-crypt failed
Error: 'git checkout' failed
git-crypt has been set up but existing encrypted files have not been decrypted
```

I've tried removing all encrypted files, as well as `.gitattributes`, committing those changes, then starting over. That hasn't worked. However, there doesn't seem to be a documented procedure for truly "starting over" in a given repo. In scenarios like this, there's not much debugging that the end user can do themselves.

Having documentation, or a built-in command to remove git-crypt from a given repo, would go a long way towards giving developers the tools they need to fix this issue rather than filing tickets for the maintainers to resolve.