Use one instance of the docs plugin

[imsingh]

If you look at the docusaurus.config.js, we have multiple instances of the docs plugin initialized. Can we do the following:

• Try to use just one instance of the docs plugin
• Use the folder structure to generate a sidebar menu

https://github.com/keptn-sandbox/new-keptn-docs-engine/blob/a4115b723125e456917b5d0281387255b6a02517/docusaurus.config.js#L102

cc @iamrajiv @vadasambar