New instructions on getting a site with demo content

[joshdentremont]

Purpose / why

As discussed in the tech call, we are removing make local and make demo. This PR removes the instructions on how to make a demo site, and adds instructions on how to add demo content to your development site instead.

What changes were made?

removed demo installation instructions page
added instructions on how to run make demo_content to get demo content in a development site

Verification

Run make starter then make demo_content to get a demo site

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?