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 https://github.com/Islandora/documentation/issues/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:

• [x] Does this PR link to related issues?
• [x] Does the proposed documentation align with the Islandora Documentation Style Guide?
• [x] Are the changes accurate, useful, free of typos, etc?

Person Merging

The person merging should ensure the following:

• [x] 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?
• [x] If pages are added, have they been linked to or placed in the menu?
• [x] Did the PR receive at least one approval from a committer, and all issues raised have been addressed?

[joshdentremont]

Thanks @rosiel that has been merged now