docs: update how to enable Postmark integration ✏️

[tomas-salgado]

Description ✏️

The updates in the PR here added the .env variable POSTMARK_API_TOKEN to two more files. This PR makes the quick update in the docs to reflect the change.

Type of Change 🐞

• Feature - A non-breaking change which adds functionality.
• Fix - A non-breaking change which fixes an issue.
• Refactor - A change that neither fixes a bug nor adds a feature.
• Documentation - A change only to in-code or markdown documentation.
• Tests - A change that adds missing unit/integration tests.
• Chore - A change that is likely none of the above.

Checklist ✅

• I have done a self-review of my code.
• I have manually tested my code (if applicable).
• I have added/updated any relevant documentation (if applicable).

[ramiAbdou]

TYSM! Completely forgot to update the documentation here. One note is that the Postmark integration still isn't working the best in development for non-colorstack.org emails, but I'm exploring alternative options.