docs: MVP plotly-express docs

[alexpeters1208]

Minimum required docs for the plotly-express plugin. Here are the outstanding items:

1. Fill out "other".
2. Document ecdf once it is implemented.

Directions for testing:

As of 7/17, everything needed for testing is baked into a release. Here's a simple testing environment using pip-installed DH.

# make new dir for testing
mkdir test-dx && cd test-dx

# create env for installs
python -m venv test-dx-venv
source test-dx-venv/bin/activate

# install some necessary things for the build
pip install --upgrade pip setuptools

# install the server, need 35.1 or 34.3
pip install deephaven-server==0.35.1

# install the plugin
pip install deephaven-plugin-plotly-express

# I need to do this to get `which deephaven` to give the correct venv version, you may not
deactivate
source test-dx-venv/bin/activate

# start the server
deephaven server