New docs extras

[alexcjohnson]

Several updates to dash, designed to enable multi-language docs generated from markdown to work better:

• 8503dfa - extra_hot_reload_paths: specify paths to be watched recursively for changes in any file, when you know that the app behavior depends on these files. I want this because I'm reading markdown files and converting them to Dash components.
• 77c15c5 - provide more granular entry points to the component generator. I already have all the metadata.json( and package.json) files for all our component packages, I just want to use these to generate non-Python backends in order to read out the docstrings.
• 9d0e821 - remove the constraint that requests_pathname_prefix ends with routes_pathname_prefix. This coincidentally just came up in Unable to load _dash-layout and _dash-dependencies from dash app running behind Nginx  #1665 but when you are serving your app behind a reverse proxy that rewrites URLs that constraint needs to be violated.

Contributor Checklist

• I have added entry in the CHANGELOG.md

[jdamiba]

These changes look good to me! 💃🏿