documentation: add BEEFY 'spec' (#12920)

* beefy: add BEEFY 'spec'
* Apply suggestions from code review

Signed-off-by: Adrian Catangiu <[email protected]>
Co-authored-by: Tomasz Drwięga <[email protected]>
Co-authored-by: André Silva <[email protected]>
Co-authored-by: Alexandru Vasile <[email protected]>
Co-authored-by: parity-processbot <>