Re-write FreeBSD instructions

[stephentoub]

The document is hard to read and follow:
https://github.com/dotnet/corefx/pull/41774/files#r334952150
cc: @carlossanlop

[carlossanlop]

Thanks @stephentoub . I'll follow up on this when I get the chance, and I'll ask the original writers of this document for their input.

[wfurt]

that would be probably me :) We tracked progress on Wiki but so much got broken by Arcade invasion. And for master I would suggest to wait until repo consolidation is done.
There is similar and obsolete version for coreclr.

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[am11]

cc @Thefrank, @sec, we can follow the format used in https://github.com/dotnet/runtime/blob/main/docs/workflow/building/coreclr/linux-instructions.md and update https://github.com/dotnet/runtime/blob/main/docs/workflow/building/coreclr/freebsd-instructions.md accordingly. Also, revisit https://github.com/search?q=repo:dotnet/runtime+path:docs+freebsd&type=code to see if there are other updates needed. FreeBSD instructions were written long time ago which predate runtime monorepo and arcade infrastructure.