docs: Linked to the 'Building on FreeBSD' section of the Unix guide
where it lists BSD specific guides.

Created a FreeBSD build guide (doc/build-freebsd.md).

Added in warning about the version of 'gdb' installed by default.

Removed the FreeBSD build instructions now that they have their own
guide (doc/build-freebsd.md).

Updated the sentence to refer to the BSD guides in the 'doc' directory
for more specific BSD build instructions.

Minor grammatical fix.