Native method guide rework

[rijulshrestha]

No description provided.

[rijulshrestha]

I've simplified the code a bit. And I had to add type="module" in the script tag for the example to work in my machine.
I belive we should move the 'Calling back into Java from JavaScript' at the end of before 'Example Walkthrough' as does not fully fall under the scope of the Native Methods guide. Pleasse suggest.

[epanholz]

I agree that we can move the "Calling back into Java from JavaScript" after the "Walkthrough". Same goes for the "Converting Parameters and Return Values" part. Please move both of these further down. Maybe we should add one main header for the example, so it looks like this:
├── Steps to implement native methods in CheerpJ
    ├── Declaring Native Methods in Java
    ├── Implementing Native Methods in JavaScript
    ├── Initializing CheerpJ with the natives option
    └── Example Walkthrough
├── Calling back into Java from JavaScript
└── Converting Parameters and Return Values

We do the same in the native libraries guide.

[epanholz]

We added the "native" prefix to all the previous native functions. Also leave it here for consistency.

[rijulshrestha]

I tried to reorder the page itself in the documentation site where currenty the native methods guide comes after the native libraries but couldn't figure out how to do it. This following order would make more sense.
├── Implementing Native Methods
├── Implementing Native Libraries