Added demo content instructions

[joshdentremont]

Purpose / why

To easily add demo content to a site template install

What changes were made?

Added new instructions to use the workbench and demo content repos with a site template install

Verification

Spin up a new site template site. Copy the instructions from this PR. Check that the workbench import worked.

Interested Parties

• @Islandora/documentation

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?

[aOelschlager]

This looks good. Thanks Josh!