Documentation re-organization

Create a better structure to make the documentation more intuitive to navigate and easier to contribute
The documentation should include:
- User Manual: how to install, how to use, best practices, etc @Xazin
- Developer Docs: how to install from source, how to contribute, architect design, how to xxx, etc @LucasXu0
- Community Docs: programs, mentorship projects, community themes and plugins @richardshiue
- Organization Docs: values, roadmaps @annieappflowy

Steps:
- [ ] Create outlines (parent nodes and subpages under)
- [ ] Re-organize the current pages based on the new outlines
- [ ] Add missing resources
- [ ] Team members should be responsible for writing product manuals for the features they built and to be built
- [ ] Send out a survey to the community for feedback and calculate the base rating for future iterations

High priority:
- [ ] Write a doc on how to self-host AppFlowy @speed2exe
- [ ] How to use TestFlight to test AppFlowy Cloud @LucasXu0