Remove all references to the mirror repo and mirror pages site.

Pages now work from this repo so we no longer need the mirror repo to build the pages site. Remove all references to the mirror repo from the documentation and change all links/references to the pages site from https://c-pathontology.gitlab.io/critical-path-ontology/ (mirror pages link) to https://c-pathontology.pages.c-path.org/critical-path-ontology/ (source pages link).

List of locations to change:

• docs/cpont-editors.md: "How do I make and see changes to these editors docs?" section
• mkdocs.yaml: site_url