BD-3726: Add more details for sending emails for SparkPost

[lydia-xie]

Why are you making this change? (required)

Add more details for sending emails for SparkPost and add link to Apple's documentation

Related PRs, issues, or features (optional)

• BD-3726

Feature release date (optional)

• N/A

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[tiffanyjlin]

do we provide any info on the UID needed? It's in the DNS records and not super straightforward. I looked up an example from the Guardian, for the domain abmail.editorial.theguardian.com:

the UID would be 8143873, so the address should be: [email protected]

[tiffanyjlin]

we typically provide the DNS records in a CSV format (not like the UI above, which was directly in Sendgrid's UI)

[tiffanyjlin]

Could we also have someone from Email Ops (Rocky's team) review? Thank you!