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

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• 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

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.

[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 😀

…

On Thu, Jan 19, 2023, 11:58 AM childish-sambino ***@***.***> wrote: @stern-shawn <https://urldefense.com/v3/__https://github.com/stern-shawn__;!!NCc8flgU!eoPVpRrtZ1WyVtZdq2JZ33FjjO0PrZ7CdIsGlCKYFxdP3YyyPLPS6ZbPiVGYEPtkJacimY7EELGcLYJzo41BIA$> Good to merge? — Reply to this email directly, view it on GitHub <https://urldefense.com/v3/__https://github.com/twilio/twilio-node/pull/895*issuecomment-1397525865__;Iw!!NCc8flgU!eoPVpRrtZ1WyVtZdq2JZ33FjjO0PrZ7CdIsGlCKYFxdP3YyyPLPS6ZbPiVGYEPtkJacimY7EELGcLYKuy5YI-A$>, or unsubscribe <https://urldefense.com/v3/__https://github.com/notifications/unsubscribe-auth/ACMZ6T2G4WR2GOSLG5RJW43WTGMHFANCNFSM6AAAAAAT7P4PGE__;!!NCc8flgU!eoPVpRrtZ1WyVtZdq2JZ33FjjO0PrZ7CdIsGlCKYFxdP3YyyPLPS6ZbPiVGYEPtkJacimY7EELGcLYLe3rl3_A$> . You are receiving this because you were mentioned.Message ID: ***@***.***>