Split the generateDocs as lib or cli

So we can use it everywhere not only in the GitHub Actions :)

A simple PoC here: https://github.com/hellodword/action/tree/patch-3

Try it with:

```sh
npx ts-node ./src/cli/main.ts features docs --owner ohhh --repo wuw ./__tests__/examples/simple/src
```

And, would it be better to implement `generateDocs` and `validate` things in [the upstream cli](https://github.com/devcontainers/cli)?

```sh
devcontainer features validate
devcontainer features docs
devcontainer templates validate
devcontainer templates docs
...
```

Two implementations always have differences.