Site site_url for mkdocs

[misilot]

Since mkdocs v1.2 site_url is now required.

• Update mkdocs.yml

Purpose / why

Bring back the clean URL's that are indexed in Google

What changes were made?

Added site_url to mkdocs.yml

Verification

Build mkdocs and see that directories are created for each page, instead of .html pages

https://www.mkdocs.org/user-guide/configuration/#site_url

Interested Parties

@Islandora/documentation

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Checklist

Pull-request reviewer should ensure the following

• Does this PR link to related issues? GitHub IO pages are broken #1853
• 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?

[rosiel]

Tested, and it works both in mkdocs 1.1.2 and 1.2.