Docs: getting started guide

[fhennig]

epic: stackabletech/documentation#237

Acceptance criteria:

• a new module getting_started exists, with an index.adoc, a installation.adoc and first_steps.adoc
• The old installation.adoc is removed.
• All the yaml and shell snippets are in the examples/code directory and can be executed as a script, to test the documentation
• versions to install are templated using the template_docs.sh script and the templating_vars.yaml file.
• Any example files that are incorporated into the getting started guide are removed from the examples directory (because there they are not tested, with the new script they will be tested).