SOGo: Add theme customization instructions and fix service configurations

[stephdl]

Brief Description

This feature request aims to improve the usability and flexibility of the SOGo integration by:

Providing clear instructions on how to customize the SOGo theme, which is currently undocumented.

Fixing the state-include configuration by including missing directories to ensure data persistence and backup consistency.

Ensuring the SOGo application service creates necessary directories at startup to avoid runtime errors.

Proposed solution

Add a dedicated section in the README explaining how users can override or customize the SOGo theme, including file paths and persistence recommendations.

Update the state-include configuration to include the required SOGo directories (e.g., for themes or runtime data).

Modify the SOGo systemd unit or entrypoint script to create expected directories (e.g., /var/lib/sogo/GNUstep/Defaults) if they are not present.

Alternative solutions

Document the customization instructions externally (e.g., in the community forum) instead of adding them to the README, though this may reduce visibility and usability.

Rely on users to manually create needed directories post-deployment, but this is error-prone and contradicts container usability best practices.

Additional context

This change would benefit system administrators who want to brand their SOGo webmail instance or apply usability tweaks, and it would also increase robustness across container restarts and upgrades.

[stephdl]

closing, the implementation is not easily possible we have many manual steps, see https://community.nethserver.org/t/sogo-new-features-configuration-template-and-access-configure-button/24906/56