docs: add explanation of GET usage in dedicated

For issue gitlab-com/gl-infra/delivery#19633 (closed)

This doc is migrated from Google Docs, with some improvements in format and update from the feedback in the last demo session.

I create a new directory named gitlab-dedicated since I see we will get more and more docs related to it. If we all agree, I will move all our docs about Dedicated (currently only one) to under this directory.