updated layout of docker installation pages in preparation for site template docs

[joshdentremont]

Purpose / why

Preparation for more docs on the site template. This is the first step to addressing #2302

What changes were made?

Updated docker installation page layout to group isle-dc and site template under "Docker" and added more folders to keep the docs separate. Also a couple small wording changes so that prereqs and intro to isle can be shared between both.

Verification

Check that the pages still load.

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?

[rosiel]

I've put a PR on your repo to add redirects and fix a broken link I found.
Other than that looks good! Approving.

[joshdentremont]

Thanks @rosiel that has been merged now