The use of "pages" as a special job name for GitLab CI to publish GitLab
pages has [been deprecated][1]. Move to using the blessed
`pages:publish:` construction instead which also allows customising the
build directory as a bonus.

[1]: https://docs.gitlab.com/ci/yaml/#pages

Closes #117

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

Closes #117

See merge request !133