Clean up the page structure

[fretchen]

With the newly added climate resilience part the general logic of the website is a bit broken. A few parts remain overarchingly true:

• The contributing.md, the disclaimer.md and the tech-faq.md keep their logic.
• However, the introduction should be redone to explain that there is now a general part and the part that is specific to climate-resilience.

Any suggestions @Maja4Dev ?