Add changelog to v3 core spec and update frontmatter

[jstriebel]

This PR

• adds a changelog for the v3.0 core spec regarding implementation relevant changes. This is supposed to help implementations that track this spec to update with the spec.
• updates the frontmatter across different pages, mostly emphasizing the current status via a warning and updating some links. I also included some feedback about the frontmatter from ZEP0001 - Core v3.0 spec for review #149.
• removes the version pinning for sphinx, as the old sphinx version does not work with current (unrestricted) versions of it's dependencies. The output of the current version conforms to the old one.

[joshmoore]

Thanks for getting this started, @jstriebel. 👍

[MSanKeys963]

Hi @jstriebel. Thanks for sending the PR.
All looks good to me, except for the comment I left above. 🚀

[jstriebel]

Also added a change for the "Editor's draft " titles: They used to be subtitles, which prevented the TOC (on the right) from expanding, since all other sections were a subpart of that subtitle. Changed it to not be a subtitle anymore, which directly shows all the elements in the TOC now.