Use readthedocs Docker image in CI

[wtraylor]

The CI step "doxygen" in .gitlab-ci.yml currently runs with the ubuntu:latest Docker image. It needs various dependencies which take time and resources during CI.

It would make more sense to use the Docker image readthedocs/build:latest because that already has almost everything packaged (except cmake), and it is also the one used by Read the Docs to build the documentation (compare the docs). So the CI can catch potential issues before the release.

Unfortunately, the "latest" (7.0) does not contain the necessary Doxygen version (1.8.16), and the pipeline fails with it. ("testing", 8.0, seems to have a bug because conda is not available, which is required to install cmake.)

As soon as readthedocs/build:latest is updated to Ubuntu 20.04, we can use it in .gitlab-ci.yml like so:

doxygen:
  image: readthedocs/build:latest
  stage: build
  before_script:
    - conda install --yes cmake
  script:
    - mkdir --parents build
    - cd build
    - cmake
      -DSTOP_DOXYGEN_ON_WARNING=ON
      -DBUILD_TESTING=OFF
      -DBUILD_DEMO_SIMULATOR=OFF
      -DBUILD_DOC=ON ..
    - make --jobs=6 megafauna_docs
  artifacts:
    paths:
      - build/docs/html/