Start with v2.6.0 beta1 (and fix images which didn't reference master branch)

[kirchsth]

• New release is created in branch release/v.2.5.0 with tag v.2.5.0.
  • all include paths in branch are updated and reference the tagged version
  • all image-urls in the *.md files are updated that they reference the tagged version
  • README.md is updated to a "Release README"
  • All changes was done "semiautomatic" (python script, ... will be added as feature/148 that it can be finished as part of #148)

This PR contains an update C4Version() to "2.6.0beta1" that all following changes are deployed with the new version