[SeasonOfDocs] QGIS install guide

[matteo]

Hi Andrew,

thanks for the document.


> I have started work on this documentation issue #3873, an install guide
> for QGIS - https://github.com/qgis/QGIS-Documentation/issues/3873
> 
> I have completed the section for Windows and would be happy to receive
> feedback, suggestions, and corrections in my google doc below before
> proceeding with the Mac and Linux installations.
> https://docs.google.com/document/d/1lJzgp9NbAxFhKTUfv9gz1_mZp38UgF-Hu_H_rUKk1yk/edit?usp=sharing If
> you would like edit access, or want to collaborate on the creation of
> this please let me know and I will give you edit access.

I made some additional comments. I'm not a Windows user, so comments and
suggestions are for a general view.

Thanks

Matteo