Docs feedback: Add experiment warning to GRIT page

• Start this issue's title with Docs: or Docs feedback:.

Problem to solve

The GitLab Runner Infrastructure Toolkit is marked as an experiment, but only in the page details. This probably doesn't mean anything to a customer unless they are familiar with our Support for features in different stages of development.

Further details

Proposal

This creates extra support burden since users typically assume (correctly) that if something is documented, it's supported. To reduce this burden, equip customers with the right info, and comply with our style, we should add the info in Documenting experimental and beta features.

Who can address the issue

Tech writer for grouprunner

Other links/references