feat: Use conda-forge for all dependencies (#7)

* Simplify environment by using conda-forge distributions for all dependencies.
   - It is sufficient to just require the Carpentries Workbench tools and all other dependencies
     will be installed from there.
* Remove the 'install-requirements' task.
* Simplify 'start' task to just run the 'serve' task.

Author: matthewfeickert