Update email_performance_dashboard.md

[wendy-p]

Updates to page - rename to 'channel performance dashboards' and added SMS section. Under new SMS section, could we replace the metrics calculation section with what is here? https://docs.google.com/document/d/1ktspVqXqOGPwetajcuiBq6r6HT0Z-EZNyhXuA_7oyeE/edit

In addition, would we be able to release this 9/22?

Pull Request/Issue Resolution

Description of Change:

Updating page now that SMS dashboard is being released.

Closes #ISSUE_NUMBER_HERE

Is this change associated with a Braze feature/product release?

• [x ] Yes (week of 9/20)
• No

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