Add namespace and using directives to AL code examples in email extension doc

[jeffreybulanadi]

All AL code blocks in devenv-extending-email.md were missing namespace and using declarations.

Changes:

• Platform interface reference blocks (Email View Policy, Email Connector) now show the real namespace System.Email; header, sourced from BCApps/src/System Application/App/Email/src/
• All user extension code blocks (enumextensions, table/page extensions, event subscribers) now declare namespace MyCompany.EmailExtension; with using System.Email;
• Completed the incomplete Email Connector enumextension snippet that was missing its outer enumextension declaration
• Converted the bare "Email Importance Enum" plain-text line into a proper ## section heading with a descriptive sentence

Verified against BCApps (System Application/App/Email):

• interface "Email Connector" -> EmailConnector.Interface.al, namespace System.Email confirmed
• interface "Email View Policy" -> EmailViewPolicy.Interface.al, namespace System.Email confirmed
• All procedure signatures match BCApps interface files exactly

[learn-build-service-prod]

Learn Build status updates of commit 8c3a9b0:

✅ Validation status: passed

File	Status	Preview URL	Details
dev-itpro/developer/devenv-extending-email.md	✅Succeeded

For more details, please refer to the build report.

[learn-build-service-prod]

Learn Build status updates of commit 39ef36a:

✅ Validation status: passed

File	Status	Preview URL	Details
dev-itpro/developer/devenv-extending-email.md	✅Succeeded

For more details, please refer to the build report.

[jswymer]

@jeffreybulanadi Thanks for the contribution! It's very much appreciated.