(PDB-551) Include a versioning policy

This patch adds a new document for outlining our ongoing software & API
versioning policy.

Signed-off-by: Ken Barber <ken@bob.sh>
(PDB-551) Some minor cleanups to the versioning policy

Signed-off-by: Ken Barber <ken@bob.sh>
(PDB-551) Reduce wording based on feedback

Reduced the amount of explicit statements based on feedback, simplified
the wording around experimental a lot.

Also provided a link to the commands versioning page.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #953 from kbarber/ticket/stable/pdb-551-versioning-policy

(PDB-551) Versioning policy