Update Source install instructions to use `make build-git`

Source install documentation includes instructions for installing Git. It has to be present to obtain the source, but needs to have the correct version too.

Proposal

Remove the Git install from the dependencies section: https://docs.gitlab.com/ee/install/installation.html#1-packages-and-dependencies, and move it to Gitaly to leverage make download-git.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information