Address #1718 menu issues.

[rosiel]

Purpose / why

Address #1718 When the menu is collapsed into hamburger-lines, the landing page shows only the "About section" menu, which looks like the whole docs are real small!

What changes were made?

Pulled the main page out to be the "About" page, on the top level of the nav menu.

Verification

Test locally

Interested Parties

@Islandora/8-x-committers @dannylamb

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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]

Built the docs, previewed in "mobile" view, and this seems to fix the issue. When you hit the docs, you see the entire TOC now.

[manez]

Resolves #1718