Replace use of envisage "contributes_to" decorator in docs

[rahulporuri]

This PR removes the use of envisage @contributes_to decorator in favor of a trait List with metadata contributes_to instead. This is being done because the next release of envisage removes the decorator entirely. Ref enthought/envisage#402

[aaronayres35]

LGTM

Orthogonal to this PR, but this files existence entirely is a bit out of place. There are preferences docs in the envisage docs that we likely want to merge with this (https://docs.enthought.com/envisage/envisage_core_documentation/preferences.html)

[rahulporuri]

Orthogonal to this PR, but this files existence entirely is a bit out of place. There are preferences docs in the envisage docs that we likely want to merge with this (https://docs.enthought.com/envisage/envisage_core_documentation/preferences.html)

You're right. These docs do probably belong in envsiage, not in apptools.

[rahulporuri]

created enthought/envisage#403