Add full help messages for all commands

notebooks/tests/animate.ipynb

notebooks/tests/assert.ipynb

notebooks/tests/dot.ipynb

notebooks/tests/eval.ipynb

notebooks/tests/groovy.ipynb

notebooks/tests/help.ipynb

notebooks/tests/load_cell.ipynb

notebooks/tests/load_file.ipynb

notebooks/tests/pref.ipynb

notebooks/tests/prettyprint.ipynb

notebooks/tests/render.ipynb

notebooks/tests/show.ipynb

notebooks/tests/solve.ipynb

notebooks/tests/table.ipynb

notebooks/tests/time.ipynb

notebooks/tests/type.ipynb

notebooks/tests/version.ipynb

src/main/java/de/prob2/jupyter/commands/AssertCommand.java

src/main/java/de/prob2/jupyter/commands/BrowseCommand.java

src/main/java/de/prob2/jupyter/commands/Command.java