Broken link to installation instructions

[patrick-nicodemus]

The readme links to the installation instructions but the link is broken.

Note also here - https://agda.readthedocs.io/en/latest/getting-started/a-taste-of-agda.html
"Make sure that you installed agda and a compatible version of the standard library"
This link is also broken.

[jamesmckinna]

In lieu of an immediate PR to the main agda documentration:

• The former link was broken by PR [ fix #1743 ] move README to doc/ directory #2184 fixing issue top-level README.agda (and Everything.agda) #1743 and needs updating to: https://github.com/agda/agda-stdlib/blob/master/doc/installation-guide.md
• Suggest that you also raise an issue about the readthedocs pages on the main Agda issue tracker