docs: Add missing alert provider group override options

[PythonGermany]

Summary

The following providers seem to support group overrides but have the documentation for its config keys missing:

gatus/alerting/provider/awsses/awsses.go

Lines 189 to 197 in 40b1576

	// Handle group overrides
	if provider.Overrides != nil {
	for _, override := range provider.Overrides {
	if group == override.Group {
	cfg.Merge(&override.Config)
	break
	}
	}
	}

gatus/alerting/provider/custom/custom.go

Lines 173 to 181 in 40b1576

	// Handle group overrides
	if provider.Overrides != nil {
	for _, override := range provider.Overrides {
	if group == override.Group {
	cfg.Merge(&override.Config)
	break
	}
	}
	}

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• Updated documentation in README.md, if applicable.

[TwiN]

Good catch