Move "Preferences in Envisage" documentation to Envisage?

[kitchoi]

This documentation section "Preferences in Envisage" looks like it belong to Envisage...?:
https://github.com/enthought/apptools/blob/addd26bac39b299c47630485e58c5649348e5a00/docs/source/preferences/PreferencesInEnvisage.rst

Envisage has a similar section: https://github.com/enthought/envisage/blob/3a7ab1c293107462e2022862ef8d3b71a20156b6/docs/source/envisage_core_documentation/preferences.rst

[corranwebster]

Yes, those pages can probably be merged in the Envisage docs.

[aaronayres35]

We will likely want to copy the relevant portions of the docs in apptools to envisage before the upcoming release. See comment: #248 (comment)