CI: adding devdeps testing and cron and workflow dispatch

[bsipocz]

Some cleanup and enhancement to the CI.

I choose to not run the devdeps job on PRs so e.g. new sphinx incompatibilities won't create failing CI status for them. However, test will run weekly on master, so incompatibilities can be picked up.

I included testing only with myst-parser and sphinx dev versions, but the job can be extended to include other dependencies.