[9.20] chg: doc: Read the Docs should always source requirements.txt from "main"
requested to merge backport-mnowak/rtd-always-resource-requirements-txt-from-main-9.20 into bind-9.20
HTTP URI of doc/arm/requirements.txt on the "main" branch is the source of PyPI packages in CI base image. To make it consistent in all maintained CI branches on RTD, use the HTTP URI as well.
This change is non-material on the "main" branch, but ensures the right packages on stable branches where for RTD we currently leverage outdated packages because we failed to update doc/arm/requirements.txt there.
Backport of MR !9630 (merged)