Spike: HTML format navigation improvements

[IgnatBeresnev]

There are multiple problems related to navigation:

• Avoid page refreshes during navigation via left side panel #2150
• Loading of the navigation tree takes too long in big projects #3133
• Dokka should be fairly usable without http server #1795

Each problem could be addressed on its own, but all of them have multiple possible solutions, and it's difficult to estimate which will be more profitable and how each solution will affect the others.

To avoid a situation where we re-write the same place three times, it might be possible to address all of these problems in one go, but it also needs to be researched and estimated.

This issue exists to track such research and answer the following questions:

Is it possible / worth it to:

• Break down navigation.html into smaller navigation-module.json, which could be loaded independently as the user opens the navigation dropdown.
  • How difficult is it to implement, effort / time estimates
• How should page refreshes be approached?
  • Vanilla JS - viability, limitations, effort / time estimates (Make not-found-version page dynamic #2738)
  • Re-write navigation to React - benefits, effort / time estimates (Offline dokka #1999)
• How to solve the offline mode issue, where the navigation menu is missing
  • Will breaking navigation.html into navigation-module.json help?
  • Wrapping navigation.html as a .js file - is it possible, what are the disadvantages / possible problems

The ideal output of this research is some summary of the findings in the comments.

The research is limited by time, ~ a week.