[Agda] References to the "old" ways of installing the agda stdlib

[Natalie Perna]

> This page seems to be out of date (and I don't know how it is
> generated). The latest README contains the following text:
>
>   -- The library comes with a .agda-lib file, for use with the library
>   -- management system.

The github.io site is published from the gh-pages branch, which was
populated by Travis CI:
https://github.com/agda/agda-stdlib/blob/master/.travis.yml
It looks like it stopped automatically updating almost a year ago
though: https://github.com/agda/agda-stdlib/tree/gh-pages