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.
Backport of core PR13372
Depends on D2461