content duplication: Getting Started and First Steps

The content in tutorials/getting_started and introduction/first_steps heavily overlaps.
Perhaps merge the hands-on PromQL walkthrough from first_steps into the existing tutorial. Delete first_steps.
Add redirect.

edit: I misfiled this, we should unify the content in introduction/first_steps and remove tutorials/getting_started. Apologies for the confusion.

Compare tutorials/getting_started and introduction/first_steps, beginning with the PromQL walkthrough and the overlapping sections. Unify the content in introduction/first_steps, remove tutorials/getting_started, and add a redirect from the removed tutorial path. Done means the unified page preserves the useful material and the old path redirects correctly.