Dash for Julia Documentation needs Sidebar for parity with Python docs

[rpkyle]

Similar to #821 , the Dash for Julia docs also require a sidebar component for searching and filtering chapters/pages of the docs. This may require also building a Julia specific index since some of the chapters and indices in the Python docs do not yet exist for the Julia documentation.

This will likely entail

• Generating a sitemap as in Python (https://github.com/plotly/dash-docs/blob/master/generate_sitemap.py)
• Loading a sidebar index (https://github.com/plotly/dash-docs/blob/master/dash_docs/run.py#L27)
• Using the sidebar component (https://github.com/plotly/dash-docs/blob/master/dash_docs/run.py#L93)