missing part of documentation

the nuklear.h file report this docs index:
```
/// ## Contents
...
/// 6. API section
/// 1. Context section
/// 2. Input section
/// 3. Drawing section
/// 4. Window section
/// 5. Layouting section
/// 6. Groups section
/// 7. Tree section
/// 8. Properties section
/// 7. License section
...
```
and the generated Nuklear/doc/index.html is the same,
but nuklear.h prototype between 6.7 and 6.8 list those sub-chapters too:
```
Tree

LIST VIEW
WIDGET
TEXT
BUTTON
CHECKBOX
RADIO BUTTON
SELECTABLE
SLIDER
PROGRESSBAR
COLOR PICKER

Properties

```
those sub-chapters are hard find if not studying the whole header.
Can you please fix the documentation?

Compare the contents index in nuklear.h with the generated Nuklear/doc/index.html and the section headings listed between Tree and Properties. Update both documentation indexes so the missing sub-chapters are discoverable, then verify that the entries and ordering match the header.

Written by the indexing model from the issue text.