Preparation: disable non-Windows GitHub Actions continuous integration

In this branch we'll be testing a theory[1] about why some
recently added `linkcheck` builder tests seem to take a longer
duration to run that some others on Windows during GitHub Actions
CI.

Even when other jobs and workflows are enabled, they should be
isolated from the Windows `main.yml` unit testing, so, to
marginally reduce cost and improve focus, this commit disables
all unrelated GitHub Actions CI (e.g. Linux, MacOS, linting, ..).

Refs:
 [1] - sphinx-doc#14046 (comment)

Author: jayaddison