Add smart card configuration and howto

What does this merge request do and why?

Add smart card configuration and howto

The current setup process for signing in with certificates or smart cards is documented in disparate places, with no centralized how-to or recommendations for configuring GDK for it. This change adds a configuration section for smartcard to gdk.yml with sane defaults, and populates the appropriate nginx and gitlab.yml sections with the required configs.

How to set up and validate locally

1. Set up and install gdk on this branch
2. Follow the smartcard.md guide in the howto added in this MR - markdown preview link
3. Ensure you can log in with a system certificate or smart card

Impacted categories

The following categories relate to this merge request:

• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.