Handbook: adjust for the various git migrations.
- prefer to install git as a package over building it oneself.
- update dates in in the introduction
- let some Subversion URLs move to the src repository, as it is the only repository with an Subversion exporter active.
- let the user install git instead of subversion for doing ports work.
Reviewed by: emaste, mat
Differential Revision: https://reviews.freebsd.org/D29450