Workflows and collections new pages

[PhilReedData]

Added new pages for collection and workflows, under 'Manual content registration', before learning paths.

I think this is the first time we have documented these features properly.

Still to add:

• New sections on the Definitions page to describe any additional fields not covered by Materials or Events
• Small graphic at the top-right of the page
• Links to these pages from elsewhere in the docs (this PR has links from the main nav and from the Manual content registration index page only)