Added WPILib installer download instructions

[amichaelyu]

Added instructions for downloading the WPILib installer to help people who might be unused to installing off of GitHub. Also fixes #1992.

[Daltz333]

This change needs to be done to the prerequisites page as well

[amichaelyu]

This change needs to be done to the prerequisites page as well

What if the offline installation preparation WPILib GitHub link was changed to direct to the more detailed guide?

[Daltz333]

If we do that, we should do it the other way around. With the WPILib guide redirecting to prerequisites page.

[amichaelyu]

If the offline pages links to the WPILib install, then we could have the more detailed instructions for running the installer, which is just a note right now.

Then another note could be added here, to say something like "If you want to create future offline installation click the Create VS Code zip to share with other computers/OSes for offline install"

[Daltz333]

I'd argue that note belongs in the WPILib setup article and not there. Since prerequisites are the details of downloading and each installer has their respective installation article.

[amichaelyu]

I think it might be better to just put it in both pages, as people might be confused by being redirected to a page called "Offline Installation Preparation".