Documentation: merge README.md into the official documentation

[bschmalhofer]

There should be a single source for the documentation and that is https://github.com/RotherOSS/doc-otobo-installation/blob/master/content/installation-docker.rst.

[bschmalhofer]

Got started with moving all documetation to the installation manual. Created dedicated sections for the advanced topic.

• [x] Complete the conversion
• [x] Proofreading
• [x] merge into master
• [x] check again when https://doc.otobo.org/manual/installation/stable/en/content/installation-docker.html was updated

[bschmalhofer]

TODO:

• [x] Check https://doc.otobo.org/manual/installation/stable/en/content/installation-docker.html again after today's changes.

[bschmalhofer]

Consider the merging of README.md as complete. Closing the issue.