publish documentation from GitHub Actions workflow

[prjemian]

Following #663 and related (nexusformat/wiki#4, #702, #868), the documentation built by a jenkins workflow running on a server at STFC-ISIS is published to the GitHub pages web site (reserved by GitHub) for the definitions repository. That GitHub pages site, where the documentation is published, redirects to our manual.

https://nexusformat.github.io/definitions redirects now to https://manual.nexusformat.org/

Recently, it has become quite simple to build and publish a repository to its gh-pages web site using a GitHub Actions workflow (and is supported by Sphinx), such as this example from the apstools repository.

Our syntax checking workflow already builds the documentation with various Python versions. An additional step (or a separate workflow) could publish after a successful build. We could add restrictions that would only publish based on criteria, such as pushes to the main branch.

It is proposed to add such a workflow here, which would let us retire the service provided by ISIS (and detect problems such as #946 as they occur). We could publish based on criteria, such as pushes to the main branch.

[prjemian]

The GitHub Actions workflow log demonstrates that the workflow is successful.