Fix Markdown indentation/lists for IIIF doc

[bencomp]

Purpose / why

I noticed that the documentation page for IIIF had *s in running text, that looked like they should have been list items. When I started editing, I noticed some other grammar, spacing and style issues.

What changes were made?

• separate list from running text with an empty line
• indent nested lists with four spaces instead of three or two
• use - for list items (instead of *)
• remove unnecessary leading and trailing spaces
• add or remove periods (.) for consistent lists
• collapse three spaces to one in the middle of sentences
• remove "Just" from "Just follow these instructions", as this may discourage readers
• rewrite the sentence linking to the IIIF Awesome List to make it a full sentence
• use HTTPS instead of HTTP in the Biblissima link

Verification

The Markdown was correctly interpreted in the VS Codium preview after my edits.

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?