Add link to Documentation from within DSpace User Interface

[tdonohue]

Description

We should provide a link to the DSpace "User Documentation" from within the DSpace User Interface. This might be simply a link in the Admin Sidebar which links back to the Learning DSpace documentation.

The purpose of this link it to provide DSpace users (especially administrators) a way to find the corresponding DSpace documentation.

This link needs to be slightly different per release. For example, the 7.x version of the UI should link to the 7.x version of the docs, while the 8.x version of the UI should link to the 8.x version of the docs. We also may want to make it possible to easily customize (perhaps the URL should be a configuration?) so that sites can link it to their own custom documentation.

Related Work

Eventually, it'd be even better if we could add more Contextual Help Tooltips which can link users directly to specific areas of the Documentation for tips/tricks. However, at this time we have very few of these contextual tooltips.

Related to #2263
Related also to #2030 (where tooltips were first created)