Create switch_homepage_to_twig.md

[DonRichards]

Purpose / why

#1787

Adds instructions they may be sorta specific to Islandora's stack on how to utilize a TWIG file for the homepage.

What changes were made?

New markdown file.

Verification

Try following the instructions provided by the new document page an adding a TWIG to load the homepage.

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?

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?