Complete Migration from ReadTheDocs to Docusaurus

[zeme-wana]

To finalise the migration from RTD to Docusaurus:

• Delete the contents of doc/read-the-docs-site, only keeping the README with a migration notice.
• Move docusaurus folder into doc folder.
• Update the GH workflows for publishing the haddock site and docusaurus site.
• Delete GH web hook and update redirects on RTD site.
• Remove mentions of read-the-docs and combined-haddock from nix code
• Update release process, mentioning how to publish Docusaurus and the Haddock site
• Update links in Docusaurus mentioning the new haddock site