Add DA-Type::Experiment label to workflow documentation

What does this MR do?

Adds the new DA-Type::Experiment label to the Developer Advocacy workflow handbook documentation.

Why is this needed?

The team is creating a new label to track experimental and R&D work, including:

• Prototypes and proof-of-concepts
• Demos and exploratory initiatives
• Innovation projects

This label helps distinguish experimental work from regular process improvements and makes it easy to track R&D efforts. cc @gitlab-da

Related issues

• gitlab-com/marketing/developer-relations/developer-advocacy/developer-advocacy-meta#1071
• gitlab-com/marketing/developer-relations/developer-advocacy/developer-advocacy-meta#1070

Changes

• Added DA-Type::Experiment to the Issue Types section in the workflow handbook
• Positioned between DA-Type::Evangelist and DA-Type::Process for logical grouping