Reorganization of migration docs and addition of video links

[ysuarez]

Purpose / why

Wanted to simplify the "migration overview page" to just list the 3 main migration options. Also wanted to create dedicated pages for the 3 migration options, so that content on each page would only focus on one migration option for clarity. Finally to add 2 new community "Islandora Workbench" demo videos.

What changes were made?

• I shortened the content of migration-overview.md and added cross links to 3 new pages...
  • migration-islandora-workbench.md
  • migration-rest-api.md
  • migration-migrate-api.md
• I added 2 new links to community "Islandora Workbench" demo videos to migration-islandora-workbench.md

Verification

Check the style, grammar, the organization, the links.

Interested Parties

@Islandora/documentation

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Checklist

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?
• Does this PR update the last updated on date on the documentation page?

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?

[manez]

Looks great!