ROX-25022: emailsender dev docs

[johannes94]

Description

Extending the dev documentation for emailsender component.

Consistency of the link depends on merges of:

• https://gitlab.cee.redhat.com/stackrox/acs-cloud-service/runbooks/-/merge_requests/106
• [RHACS] Added ACSCS email integration openshift/openshift-docs#77408

Checklist (Definition of Done)

• Unit and integration tests added
• Added test description under Test manual
• Documentation added if necessary (i.e. changes to dev setup, test execution, ...)
• CI and all relevant tests are passing
• Add the ticket number to the PR title if available, i.e. ROX-12345: ...
• Discussed security and business related topics privately. Will move any security and business related topics that arise to private communication channel.
• Add secret to app-interface Vault or Secrets Manager if necessary
• RDS changes were e2e tested manually
• Check AWS limits are reasonable for changes provisioning new resources
• (If applicable) Changes to the dp-terraform Helm values have been reflected in the addon on integration environment

Test manual

TODO: Add manual testing efforts

# To run tests locally run:
make db/teardown db/setup db/migrate
make ocm/setup
make verify lint binary test test/integration

[johannes94]

/retest

[openshift-ci]

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