docs: add relevant Refer/Say/ssml links to upgrade guide; formatting

[stern-shawn]

Fixes

Updates the UPGRADE.md doc to provide docs links to relevant resources for changes that could be confusing to customers. Fixes a few markdownlint warnings.

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added the necessary documentation about the functionality in the appropriate .md file
• [ ] I have added inline documentation to the code I modified

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[childish-sambino]

@stern-shawn Good to merge?

[stern-shawn]

I'm happy with it if you are! Happy to see that break_ is transitioning to break 😀

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