feat(mkdocs-docs): do not use special "pages" job name

The use of "pages" as a special job name for GitLab CI to publish GitLab pages has been deprecated. Move to using the blessed pages:publish: construction instead which also allows customising the build directory as a bonus.

You can see the modified jobs running in https://gitlab.developers.cam.ac.uk/uis/devops/user-notify/infrastructure/-/pipelines/757944 which generated a pages:deploy job showing that GitLab recognised the pipeline as a "publish GitLab pages" job.

Closes #117 (closed)