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.
