👷🏼 Remove publish-gh-pages workflow

[ccamel]

Idea

Remove the publish-gh-pages workflow that currently handles the deployment of the ontology's documentation site to GitHub Pages. Following the implementation of issues #76 and #111, the documentation will now be generated in markdown format and subsequently integrated into the OKP4 documentation website.

Todo

• Remove any reference to https://ontology.okp4.space/
• Remove workflow publish-gh-pages
• Remove branch gh-pages and associated deployment branch gh-pages is now used to publish the ontology sources only (not the documentation - cf. 🚀 Ontology Deployment #214)