GSoC: Update and include wiki content

[jrm]

Summary

Much of the information on https://wiki.freebsd.org/SummerOfCode and
this page overlap. Include all of the information here and retire the
wiki page.

While here, update the formatting so that it matches what has been
requested by doc committers in the past.

[jrm]

Sergio, thanks again for the quick review. As I said in our private conversation, I requested a quick review because we would like to have this page updated before the office hours tomorrow. If there are tweaks to be made, I can make them starting Thursday.