[bdbarnett]

Adds a Style workflow that calls the organization's shared style check on every pull request. It fails on a Python file without the three SPDX lines, a Sphinx role or field in a docstring, a README or docs page that links to the organization's shared-tooling repository, and a README badge row or status banner. On a pull request it reads only the changed files; run it by hand to check everything.

One page failed: docs/guides/regenerate-the-pages.md had a clone command with a link to that repository. That URL is gone; the guide still says to put the generator's repository in a folder called dotgithub. The guide as a whole describes work that happens in that repository, so it may belong there instead; I left it where it is.

I ran the check over every tracked file and it now finds nothing. The workflow calls the shared check at publishing-v20, which doesn't exist yet, so it will fail to start until that tag is cut. Merge this after the tag.