Use a single directory tree for writing and rendering the docs

[ptgott]

In order to migrate our previous docs site to Docusaurus, we retained the previous docs site's git submodule configuration in the content directory, and run a script to move files from content to directories that Docusaurus uses for generating docs pages.

There are several issues with this approach:

• Because the docs content that we edit is not in the directory tree that the dev server watches, hot reloading is untenable in local dev environments: Allow hot reloading on the dev server #129
• There can be unexpected side effects of moving files between directories while building the site, e.g., Relative links in the Upcoming Releases page can break docs builds #128
• Makes it more difficult to onboard docs site contributors, since this custom functionality is a wrapper over the way Docusaurus is meant to work.