Viewers method

[rosiel]

Purpose / why

The method of choosing viewers described here is no longer used in the starter site.

What changes were made?

• explains the new system
• lists how to change viewers for a whole media type, or for individual media.
• removes the "look how painful this is" laundry list of what used to happen.

Verification

• does it accurately document the new starter site mechanism?

Interested Parties

@DonRichards

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?