There's a lot of confusion between compiling Godot and exporting projects.
This aims to resolve part of the confusion by cross-linking other pages
for each platform.
* Improve the Exporting for the Web section.
Add "Export Type" options (threads, gdnative, regular).
Add warnings about "secure contexts" limitations.
Mentions threads and GDNative requirements.
Mention WebRTC as a networking API.
General refactor.
* Update compiling for the Web.
Bump emscripten requirements, mention threads, gdnative and editor
build.