handbook: Split off HTTP mirrors into a separate section

Web browsers are deprecating support for FTP; thus, having
an interleaved list of FTP and HTTP links isn’t particularly
user-friendly.

Note that there is another list of mirrors in the "eresources"
chapter, but let's clean up this one first; I also have a followup
patch that cleans up rsync.

Reviewed By: debdrup
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D33952