add examples and complete the documentation (dragoon/komodo!166)

## changelog
- _semi\_avid_, _kzg_ and _aplonk_ examples have been added
- the `fs` module has been hidden behind an `fs` feature
- the `conversions` module has been properly hidden behind the `test` config feature
- the documentation has been completed
- some error messages have been improved

> **Note**
>
> the documentation of aPlonK has been left as-is for now