improve category documentation

I think this should explain this more easily for newcomers.
integrate external PNGs into the CSS
make tree icons independant of pathinfo feature

icons for files and directories in the tree view should work regardsless
of the gitweb config option

```perl
$feature{'pathinfo'}{'default'} = [1];
```
Merge remote-tracking branch 'origin/improveDoc'

* origin/improveDoc:
  improve category documentation
Merge remote-tracking branch 'origin/inline_pngs'

* origin/inline_pngs:
  integrate external PNGs into the CSS
Merge remote-tracking branch 'origin/universal-tree-icons'

* origin/universal-tree-icons:
  make tree icons independant of pathinfo feature