Add environment variable configuration documentation for AWS SES email setup

[isaiahiruoha]

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Closes: #26700

Added documentation for configuring AWS SES via environment variables in the Email Configuration How-To Guide.

Currently, the AWS SES email configuration documentation only shows setup through airflow.cfg modifications, while other providers such as SendGrid document both airflow.cfg and environment variable configuration methods. This creates an inconsistency in the documentation and limits users who prefer environment variable configuration.

Changes:

• Added environment variable configuration section for AWS SES in the Email Configuration How-To Guide
• Maintained consistency with other provider documentation (e.g., SendGrid) by showing both configuration methods

Testing:

• Verified documentation formatting and clarity
• Confirmed environment variable names align with Airflow's configuration standards

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