Update using-rest-endpoints.md

[manez]

Purpose / why

Updates for the docs sprint. Adds link to Drupal docs on RESTful API, pulls out Authorization content to its own page.

What changes were made?

New link, new page based on cut content, new menu entry for the new page.

Verification

Build & review, check the link, check against the sprint spreadsheet

Interested Parties

@Islandora/documentation, or, if unsure, @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?