docs: Added upgrade notes to readme for v8

[breautek]

Platforms affected

Docs

Motivation and Context

Added v8 section to upgrade notes.

Description

Also added sub-sections to make it more identifiable when notes start for a new version, and
reworded or reformatted pieces to make it more readable in the editor or shorter in markdown text.

Testing

VSCode markdown preview.

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary