Update MicroSite (GH Pages) for 2.x

[ascheman]

As a user of the new modularized HSC I would like to see proper documentation on the GitHub pages site.

• Fix errors and missing links (in particular xrefs).
• Re-factor development pages
  • Start with the Intro
  • Add the ChangeLog.
  • Separate design decisions
  • Show publication of new releases
  • Explain document builds
• Refer to live code where ever possible.
• Drop kbd-Macro (replace by HSC).
• Unify local publication build and GH pages generation.
• Use development branch for all (external) page references.
• Don't use numbering in Architecture sections.