Adds definition for "IDA" to Glossary page.

IDA is Independent Deployed Application

https://github.com/edx/edx-documentation/issues/1917

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   1. [DOC-XXXX](https://openedx.atlassian.net/browse/DOC-XXXX)

If the release date of a feature is known or estimated, provide it to give reviewers guidance on turnaround time.

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      1. Reviewers

Possible roles follow. The PR submitter checks the boxes after each reviewer finishes and gives :+1:.

• [ ] Subject matter expert:

• [ ] Subject matter expert:

• [ ] Doc team review (sanity check, copy edit, or dev edit?): @edx/doc

• [ ] Product review:

• [ ] Partner support:

• [ ] PM review:

FYI: Tag anyone else who might be interested in this PR here.

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   1.  

      1. Testing

• [ ] Ran ./run_tests.sh without warnings or errors

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      1. HTML Version (optional)

• [ ] Build an RTD draft for your branch and add a link here

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      1. Sandbox (optional)

• [ ] Point to or build a sandbox for the software change and add a link here

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      1. Post-review

• [ ] Add a comment with the description of this change or link this PR to the next release notes task.

• [ ] Squash commits