Wording and structural improvements

[mxmehl]

Some minor updates to improve the website:

• Projects page (see Move Projects to a separate page #53)
• First section: use the standard statements, replace misleading setences
• Second section
  • Headline: "Shared Challenges, Shared Solutions"
  • Max. 3 paragraphs, one quote/key sentence
• Community page
  • Mention incubation process for bringing a new project or contributing to existing ones (probably as last section): "Contribute to or bring projects". Independent of memberships
  • Long-term: "Convince your organization" for the join as member section ( --> split off to separate issue)