Add mergePlugins to attributes in devfile reference and fix minor issues in doc

[amisevsk]

What does this PR do?

Adds documentation for eclipse-che/che#17785 to explain new devfile attribute. Also fixes minor issues in the doc:

• Add asyncPersist to devfile reference, as it was previously missing.
• Fix a vale error on TODO in the doc
• Fix indentation on a yaml code block.

What issues does this PR fix or reference?

Required for eclipse-che/che#15373

Specify the version of the product this PR applies to.

7.19.x

PR Checklist

As the author of this Pull Request I made sure that:

• vale has been run successufully against the PR branch
  • Vale lists 12 errors on the devfile reference, but they're all spurious (using "zip" in a code sample is an error, but it's the devfile spec not prose).
• Link checker has been run successfully against the PR branch
• Documentation describes a scenario that is already covered by QE tests, otherwise an issue has been created and acknowledged by Che QE team
  • ???
• Changed article references are updated where they are used (or a redirect has been set up on the docs side):
  • Dashboard branding.constant.ts + product.json
  • Chectl constants.ts

Note: I wasn't able to check xref links fully as I ran into issues starting a workspace from the devfile on Hosted Che.

[yhontyk]

LGTM