Mention Mirador where appropriate.

[rosiel]

Purpose / why

Issue: #2163
We have another viewer now, mirador. I mentioned it where it seems to fit within the existing docs.

What changes were made?

• list mirador as a viewer option (file_viewers.md)
• list mirador as an IIIF compatible option (iiif.md)
• list mirador as a module we release with semantic versioning (versioning.md)
• list mirador as a module we release by making releases in github (releasing-islandora.md)

Verification

Please check for bad links and typos! 😄

Interested Parties

• @Islandora/documentation
• @Islandora/committers

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?