Update content_types.md

[joshdentremont]

Updated instructions for new content types to include mandatory fields and to remove steps that are no longer necessary.

Purpose / why

It was not clear to me that the member of field was mandatory, but without it the nodes are not considered Islandora objects.

What changes were made?

Added info on mandatory fields (model and member of) to make it clear that both of those fields need to be included in any custom content types.

Also removed instructions for updating context that is no longer needed. Indexing happens on all nodes that are considered Islandora objects, and as long as they have the member of field they will be considered islandora objects.

Verification

Create a new content type with the member of field included. Add an object using that content type. Check that the object is in Fedora.

Interested Parties

• @Islandora/documentation
• @Islandora/committers

Checklist

Pull-request Reviewer

Pull-request reviewer should ensure the following:

• Does this PR link to related issues?
• Does the proposed documentation align with the Islandora Documentation Style Guide?
• Are the changes accurate, useful, free of typos, etc?

Person Merging

The person merging should ensure the following:

• Does mkdocs still build successfully? (This is indicated by TravisCI passing. To test locally, and see warnings, see How To Build Documentation.)
• If pages are renamed or removed, have all internal links to those pages been fixed?
• If pages are added, have they been linked to or placed in the menu?
• Did the PR receive at least one approval from a committer, and all issues raised have been addressed?