Automate interactive API site generation and move to stacspec.org

[cholmes]

Right now our online interactive API docs live at swaggerhub, and must be updated manually. We should automate the process, and move the output to stacspec.org instead of swaggerhub.

• Find right tooling to generate the API website from the openapi docs.
• Migrate output to stacspec.org/api
• Automate the build to automatically produce the website and deploy it to stacspec.org

[cholmes]

Done! Using redoc, and now available on https://stacspec.org/STAC-api.html

Didn't actually need to use continuous integration, the tool picks up the open api docs direct from the repo with javascript

[m-mohr]

Merging radiantearth/stac-site#19 needs to be done after the 0.7 release, therefore re-opened.

[cholmes]

Closing, as radiantearth/stac-site#19 is merged.