Migrate from `gem5.org/dist` to `dist.gem5.org/dist`
Documentation and the download component uses data from `gem5.org/dist`. This is to be deprecated. `dist.gem5.org/dist` should be used instead (the latter is hosted on Google Cloud services).
Documentation (including the `learning-gem5` book) and download components point towards the the old URL.
Note: this transition should not happen until the gem5 Google cloud service is fully deployed and funded from Google research credits.