Major Update: Use mkdocs, Material Theme, GitLab Pages, reorganize docs [MNG-2202]

This converts our technical docs at https://doc.opencraft.com/ to match the style of our handbook and private docs, with cross-links to those other docs.

This also takes the "deployment docs" which were published separately at https://openedx-deployment.doc.opencraft.com/ and publishes them here, within our unified "technical docs".