doc: drop version numbers from end of Cask names

Following up on #2659, #3175, and #3255, further steps toward an algorithmic
derivation of Canonical Name from App Name.

Rationale (just as before)
- encodes the existing practice for the great majority of Casks
- such terms are not helpful in search
- for deeper goals see #2659

Enforcing this rule would affect 4 current Casks: `gpower3.rb`, `marked2.rb`, `airmail-beta.rb`, `logmein-ignition-beta.rb`

Pinging @vitorgalvao.  Here is the next round of proposed naming rules.

I can only find two exceptions where a trailing number differentiates a product from different vendors: iterm2 and pgadmin3.