DM-37525: Add openapi generator extension

[jonathansick]

This PR provides support for embedding a Redoc site for FastAPI apps and other projects that have OpenAPI specifications.

• The documenteer.ext.openapi Sphinx extension can generate the OpenAPI specification during the Sphinx build by running a user-specified function. See the docs at https://documenteer.lsst.io/v/DM-37525/sphinx-extensions/openapi.html
• The documenteer.ext.openapi and sphinxcontrib.redoc extensions are included by default with the Documenteer user guide configuration (documenteer[guide])
• Users can configure both the openapi generator and redoc extensions by setting a [project.openapi] table in the documenteer.toml configuration file. See the docs at https://documenteer.lsst.io/v/DM-37525/guides/openapi.html
• The docs advocate setting up a stub page to add the redoc HTML page to the Sphinx toctree; this stub gets overwritten by the sphinxcontrib.redoc extension. This solution removes the need to an API landing page.

For a working demo see https://github.com/lsst-sqre/squarebot/pull/18 and https://squarebot.lsst.io/v/DM-37525/index.html

[jonathansick]

Whoops, looks like I broke it. I'll check this.

[jonathansick]

https://squarebot.lsst.io/v/DM-37525/api.html is working again. I'm still not sure why the stub didn't get replaced; I haven't been able to replicate it.