Improvements regarding documentation theme

[aepfli]

Description

Currently our documentation theme is residing within keptn-sandbox. As it is somewhat matured it is time to move it from keptn-sandbox to Keptn.

Furthermore, we made some changes within this repository, which do make sense to store within the theme:

• GitHub action for releasing-docs
• partial for version dropdown
• etc.

Additionally we need to clean up the theme repository:

• removing existing documentation
• check which files are actually needed (no need for a netlify configuration etc)

Important

While moving, it should be no issue for the existing pull requests, etc. The transition should be as smooth as possible.

Besides this, we also want to establish basic versioning, so that renovate will handle the creation of update pull requests. Etc.