update deprecated docusaurus configs and local development instructions

Some of the configuration options that were likely valid in docusaurus 2.0.0-alpha.56 are now deprecated and invalid in 2.0.0-beta.3:

• homePageId has been deprecated in favor of adding slug: / in the desired Markdown file (see https://github.com/facebook/docusaurus/pull/3228)
• themeConfig.navbar.links is now themeConfig.navbar.items (see https://github.com/facebook/docusaurus/pull/2971)

Currently, the "Docs" slug in the top navbar (https://mergetb.org/docs/) points to https://mergetb.org/docs/experimentation. I think it would be more straightforward if "Docs" points to https://mergetb.org/docs/overview instead (but can update this to whatever people think is reasonable).

Additionally, I've added some extra instructions on getting a local development server running.