Fix release script and instructions

[ptitzler]

Closes #2840

What changes were proposed in this pull request?

• Update Makefile to properly detect whether the local source code is associated with an official release
• Update release documentation (remove obsolete instructions and apply minor cosmetic changes)
• The release script will now build and publish the release images during the release process as a single step

How was this pull request tested?

• Ran make publish-container-images using a customized Makefile version
• Reviewed output of make docs

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[kevin-bates]

This looks great. Just had the one comment.

[kevin-bates]

LGTM - thank you.

[lresende]

the checkouts are handled before this, on the main driver...
you just need to build the images here

[akchinSTC]

I think up to this point, the current branch of the repository under build/release/elyra would be set to main still. So any image built without these checkouts would be built using the dev tag

[akchinSTC]

@lresende - if youre good with this im going to merge