add churn red flags to customer success guide

[caitlinadian]

Changes

Adding some common red flags for customer churn and link to guidance on how to re-engage before it's too late!

Checklist

• I've read the docs and/or content style guides.
• Words are spelled using American English
• Use relative URLs for internal links
• [ not for this one on superday, but i will in the future! ] I've checked the pages added or changed in the Vercel preview build
• [ n/a ] If I moved a page, I added a redirect in vercel.json