Document batch upload children.

[rosiel]

Purpose / why

We had a discussion on Slack that the batch upload children is undocumented.

What changes were made?

New section on Paged Content page, describing the feature.
And a bit of editing the Resource Node page.

Verification

Is true/valid? Do links work?

Interested Parties

Replace this text - Name some folks who may be interested, if documentation related mention @Islandora/documentation, or, if unsure, @Islandora/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?

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?

[simonhm]

I confirm the document for "Batch Upload Children" is useful. I also checked those files changed. They are free of typos.
Thanks!
Simon.

[simonhm]

The hyphen in "Re-order" looks weird to me. Shouldn't it be "reorder", should it?