chore: restructure README

[BethGriggs]

Hey, I just made a Pull Request!

I figure there's some prominent information in the main README that is quite specific to community plugin maintainers. I've opened this PR to restructure the README slightly in effort to make the information more consumable for end-users and potential plugin contributors.

I've not removed any information. But, a summary of adjustments:

• Change tense of wording now that the main migration of plugins from backstage/backstage has occurred.
• Move the list of migrated plugins to a collapsed section.
• Move migration steps to the bottom of README as this information is the most specific to community plugin maintainers.

:heavy_check_mark: Checklist

• [ ] A changeset describing the change and affected packages. (more info)
• [ ] Tests for new functionality and regression tests for bug fixes
• [ ] Screenshots attached (for UI changes)
• [X] All your commits have a Signed-off-by line in the message. (more info)