handbook/mirrors: Update information on Official Mirrors

In Chicago, Illinois, there are now two sites:

• New York Internet / https://www.nyi.net/
• Your.Org / https://your.org/

Thank you to both companies for your generous sponsorship. Your support
is greatly appreciated.

Additionally, there is a new service hosted on most of the mirrors,
which is our web frontend for Git, https://cgit.freebsd.org/.