Read the Docs should always source requirements.txt from "main"

Read the Docs should always source requirements.txt from "main"

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packages on stable branches where for RTD we currently leverage outdated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.