[Delivers #42931479] Add mechanism for configuring and outputting context help in frontend

This doesn't need to be merged as we haven't received too much detail of the help content structure - but would be good to get this mechanism reviewed.

I've added a new config/help.yml that offers the url to the help content and mappings from sections in our frontend to their respective path on the external site.

There's an initialiser help.rb that adds an API via the module ArchivesSpaceHelp.  This loads the yml config on application startup and offers getters to the topic mappings, url etc.

I've focused on the Accession form initially.  There are some changes to generic templates that allow subrecords to insert their help link automatically... and there are a few links that needed to be inserted manually e.g. Accession's Basic Information and Deaccession Date.  The help link will only be inserted if there's a mapping for the topic/form section.