Update contributing.md for Windows contributors

[MCBoarder289]

Went through the steps on my Windows development machine and ran into errors (below). Discovered that this was likely Windows specific and wanted to simply update this so others can avoid similar issues.

Also updated some of the bash commands so that they highlighted having an activated virtual environment for consistency.

Complete output (1 lines):
A GDAL API version must be specified. Provide a path to gdal-config using a GDAL_CONFIG environment variable or use a GDAL_VERSION environment variable.
----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

Please uncomment this block and take a look at this checklist if your PR is making substantial changes to documentation/impacts files in the doc directory. Check all that apply to your PR, and leave the rest unchecked to discuss with your reviewer! Not all boxes must be checked for every PR :)

If your PR modifies code of the plotly package, we have a different checklist
below :-).

Documentation PR

• I've seen the doc/README.md file
• This change runs in the current version of Plotly on PyPI and targets the doc-prod branch OR it targets the master branch
• If this PR modifies the first example in a page or adds a new one, it is a px example if at all possible
• Every new/modified example has a descriptive title and motivating sentence or paragraph
• Every new/modified example is independently runnable
• Every new/modified example is optimized for short line count and focuses on the Plotly/visualization-related aspects of the example rather than the computation required to produce the data being visualized
• Meaningful/relatable datasets are used for all new examples instead of randomly-generated data where possible
• The random seed is set if using randomly-generated data in new/modified examples
• New/modified remote datasets are loaded from https://plotly.github.io/datasets and added to https://github.com/plotly/datasets
• Large computations are avoided in the new/modified examples in favour of loading remote datasets that represent the output of such computations
• Imports are plotly.graph_objects as go / plotly.express as px / plotly.io as pio
• Data frames are always called df
• fig = <something> call is high up in each new/modified example (either px.<something> or make_subplots or go.Figure)
• Liberal use is made of fig.add_* and fig.update_* rather than go.Figure(data=..., layout=...) in every new/modified example
• Specific adders and updaters like fig.add_shape and fig.update_xaxes are used instead of big fig.update_layout calls in every new/modified example
• fig.show() is at the end of each new/modified example
• plotly.plot() and plotly.iplot() are not used in any new/modified example
• Hex codes for colors are not used in any new/modified example in favour of these nice ones

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
• For a new feature, I have added documentation examples in an existing or
  new tutorial notebook (please see the doc checklist as well).
• I have added a CHANGELOG entry if fixing/changing/adding anything substantial.

[MCBoarder289]

Been a user and big proponent of Plotly and would like to start contributing! When setting up my environment I ran into a couple issues being on a Windows machine, so I thought it best to make those changes here. Open to hearing any feedback!

Also, not sure why one of the checks didn't pass - doesn't look related to this markdown file itself?

[nicolaskruchten]

Thank you for this PR, and I apologize it took so long to review and merge it :)