Advertise features in README and docs

What does this MR do?

Make some features of DORiE clearer in the docs and the README.md, as suggested in the JOSS paper review.

• Advertise unstructured grids, adaptive grid refinement, and parallel execution in the "Feature Overview" of the user docs. Include caveats and extend the "Available Models" table with this information.

  See https://github.com/openjournals/joss-reviews/issues/2313#issuecomment-652652512 for the original suggestion.

• Suggest using the DUNE environment Docker image for code development in the README.md.

  See https://github.com/openjournals/joss-reviews/issues/2313#issuecomment-647818129 for the original suggestion.

Is there something that needs to be double checked?

No.

Can this MR be accepted?

• Added/Updated documentation
• Pipeline passing
• [ ] Squash option set
• Delete branch option set
• [ ] Added entry to CHANGELOG.md

Assignee: If the Squash option is set, check/update the commit message right before merging!

Related issues

Closes #