Update guidebook's pages with information about Copier

As now https://gitlab.developers.cam.ac.uk/uis/devops/gcp-deploy-boilerplate/-/merge_requests/40 is merged to long-live 'copier' branch and this one uis/devops/webapp-boilerplate!76 (merged) will also be eventually merged to the 'copier' branch it is needed to actualize documentation an "how-to"s.

It should contain:

• Updates for pages like this (how to create a new app) https://guidebook.devops.uis.cam.ac.uk/en/latest/howtos/create-a-new-app/
• How to migrate existing projects (created with cookiecutter) to copier
• How to update the copier-derived project with new version of template