Add Open Data Hub image refresh instructions

[ptitzler]

Closes #2844

What changes were proposed in this pull request?

Adds a new topic to the developer guide that outlines how we update the Open Data Hub Elyra image stream.

How was this pull request tested?

• Review output of make docs

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[elyra-bot]

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[kiersten-stokes]

Found a small typo below. Also, it's slightly unclear to me when a version change to the base container image is required, but that could be my ignorance and not a fault of the PR 🙂

Other than that, everything make sense and is working for me!

[shalberd]

Very nice, thank you.

[ptitzler]

Your changes look good. We should be ready to merge once Kiersten has approved the new additions.