Update collection.md

[eldonquijote]

Added links, removed redundant text, reworded sentences for clarity

Purpose / why

Goal: Explain the concept of Collections in Islandora, with reference to bulk management and the interaction of Islandora Defaults.

What changes were made?

Added internal links and links to Drupal documentation, removed redundancies, reworded

Verification

Interested Parties

@Islandora/documentation

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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?