Simplify github workflows

[ma595]

There are currently three separate workflows doing similar things. Some simplification is needed.

• python-ci
  This builds all detectree2 dependencies and runs pytest from the root directory, executing a different set of tests.

• docker-test
  This builds on top of a docker container with detectree2 installed in order to speed up execution (the detectron wheel is not built). It only supports CPU execution. A single integration test is executed requiring a dataset to be cloned from a separate repo (not documented) detectree2-data.

• sphinx-ci
  This builds detectree2 again for the documentation.

Current issues:

• Redundancy in build - two workflows doing identical things docker-test and python-ci.
• both workflows do style, static type checking, import checks.

python-ci:

• GDAL version is explicitly set despite existing in setup.py.
• Unnecessary pip installs.

docker-test:

• --continue-on-error flag is set in flake8 and integration test.

See #38 for more on workflow dependencies.