Add copyright line to mkdocs.yml

[highermath]

Purpose / why

Add lines to footer to facilitate linking to resources.

What changes were made?

Added copyright line to mkdocs.yml

Verification

_These two dummy lines should appear in the footer above the Made with Material for MkDocs line
Some info about contributing can go here!
Change this in the copyright line in mkdocs.yml

Note

The text must be changed to something useful before pushing to production.

Interested Parties

*@Islandora/8-x-committers

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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?