Improve the navigation documentation markup

[timothyqiu]

Mainly:

• Use string literal (``text``) only when referring to something code related.
  • Many are replaced with **.
  • Some unnecessary ``something_not_code`` are turned into normal something not code.
• Adds missing `` quotes for some function/signal names and code snippets.
• Adds some ref links for class names when they first appear.
• In the debug tool article, "NavigationProcess" and "PhysicsProcess" are turned into "Navigation Process" and "Physics Process" since it's how they are displayed in the editor.

[skyace65]

@smix8 it's your section of the documentation, want to review this?

[skyace65]

Thanks!