Fix errors in documentation

[rfuehrer]

The documentation contains errors (misspelled words, missing or misleading links, changed information). The errors must be corrected in order to increase acceptance by the users and make it easier to get started.

[rfuehrer]

wip: branch rfuehrer/fix-docu

[rfuehrer]

in the existing documentation I notice the following circumstances in particular:

• there are links to files in the master branch which have nothing in common with the current state of the dev branch
• no installation instructions are provided that allow a quick start (and thus a high acceptance) - this may be due to the fact that it is not always obvious which state of the branches is still valid
• files are provided in the default branch, which run against an incompatibility error in an early phase of the installation (composer binary)
• no CI/CD pipelines are described which test an adaptation for its integration capability and are considered a prerequisite for PR