Instructions to convert isle-dc to site template

Purpose / why

In order to move away from having to support 2 Docker install options we need instructions to convert an isle-dc site to a site template site (https://github.com/Islandora/documentation/issues/2273)

What changes were made?

Added a page to describe what needs to be done to convert a site from the isle-dc Docker setup to the Site Template one

Verification

These instructions worked for me, but my site didn't have a lot of customizations. It would be good for someone else who has an isle-dc site in production to try this and see if anything else needs to be added.

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?

Islandora / documentation