Consolidate buildkit requirements (basics/requirements.md, tools/buildkit.md)

Created by: totten

1. I believe the list of system requirements in requirements.md and buildkit.md were originally copied from the same place, but they've drifted apart arbitrarily. This reconciles them by:

   • Copying some of the details in buildkit.md over to requirements.md
   • Removing the list from buildkit.md
   • Replacing with a hyperlink and a blurb about the importance of system requirements

2. When reading the "Generic" section, my eyes glazed over at the list of requirements. There are really just two instructions (git clone... and ./bin/civi-download-tools). Considering that civi-download-tools has some diagnostics built-in (and is patch-welcome for more), it seems to belabor the point by reproducing the entire list under buildkit.md#other-platforms.