Split docs for GLEX and experimentation.rb

What does this MR do?

Adds documentation for Gitlab::Experiment (GLEX), and splits the index into two paths for now.

Related to #323991 (closed) which is the follow-up to resolve a few things here.

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because documentation.
• Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content