Dash for R Documentation needs Sidebar for parity with Python docs

[HammadTheOne]

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

[Marc-Andre-Rivet]

• Generate sitemap like in Python (https://github.com/plotly/dash-docs/blob/master/generate_sitemap.py)
• Load sidebar index (https://github.com/plotly/dash-docs/blob/master/dash_docs/run.py#L27)
• Use the component (https://github.com/plotly/dash-docs/blob/master/dash_docs/run.py#L93)

The sitemap is only URLs? Is there more to this? Meta?