[Agda] installing agda

[Manuel Bärenz]

> In case there are still installation difficulties, you might want to
> check out the online version of Agda at
>
>     https://agdapad.quasicoherent.io/.

This is fantastic! What software is this? Can it be embedded easily in
an existing page?

How about making an "official" server that is linked from the agda docs?
It would be great to have this linked so beginners find it quickly.