Feature request: Allow offline mode for PWA

[Meir017]

from KirillOsenkov/SourceBrowser#239

and KirillOsenkov/SourceBrowser#245 (comment)

we should be able to serve the index as a static file and have everything run in the browser, this will remove the need to have a backend and simplify the setup to allow any C# repo to add a source-indexer to their public docs for simple navigation (maybe some integration with docfx and others?)