gh-pages: specification version lists on landing page

[ralfhandl]

Part of #3576

• List of Arazzo and OpenAPI Specification versions on landing page
• Fix warnings for some registry entries
• Fix error for broken RSS feed - do we want an RSS feed?

This is a preview of the adapted landing page: https://ralfhandl.github.io/OpenAPI-Specification/

This is a preview of the RSS feed: https://ralfhandl.github.io/OpenAPI-Specification/rss/feed.xml

[handrews]

@ralfhandl it looks like you've made quite a few changes compared to the linked preview- could you please post a new rendering?

[ralfhandl]

a new rendering

The index.html on https://ralfhandl.github.io/OpenAPI-Specification/ is produced by the same code as in this PR. Only difference is that my gh-pages already hosts 3.0.4 and 3.1.1, so they are also listed on the landing page.