ug2html.py: Removed functionality to copy tool docs since we don't have them anymore (#1754)
List DateTime library in User Guide.

This way DateTime library docs are included in User Guide distribution which fixes #1741.

Other related changes:
- Enhance time format docs a little.
- Update ug2html.py to automatically recompile library docs.
- Update lib2html.py to ease recompiling all docs programmatically.