(nl) various improvements to dutch translation

• (nl) dutch/devel/website/translating.wml: various minor improvements
  • less 'moet' (must), more active writing such as in the original english version
  • some linguistic fixes ('kunt u uw vertaling beginnen invoegen in')
  • stay more true to the english version ("which you'll need" -> welke u nodig heeft ipv welke nodig zijn)
• (nl) dutch/international/dutch/index.wml: devel/website/translating is available in dutch: Remove mention that it's still only available in english.
• (nl) dutch/intro/help.wml: minor language improvements
• (nl) dutch/intro/help.wml: change 'op te dragen' to 'instrueren': instrueren sounds less obligatory
• (nl) fix "zinnens zijn" into "van plan zijn" in:
  • dutch/devel/website/translating.wml
  • dutch/events/checklist.wml
  • dutch/ports/powerpc/inst/pmac.wml
  • dutch/women/mentoring.wml