Add support to populate pre-exisiting markdowns files (documentation)

[iamutkarshtiwari]

Is your feature request related to a problem? Please describe

Our project has several markdown documentations that were (and are being) manually added for developers to reference. These markdowns more elaborate than the regular API docs generated by Dokka and act as a supplementary information. The goal is to make Dokka able to populate these markdowns as well so that they can be rendered alongside on the same GitHub Pages as the API Docs autogenerated by Dokka.

Expectation

The Index on the left hand side should be segregable between API Docs auto-generated by Dokka and a custom section to host these markdowns.

Describe alternatives you've considered
For now, we'll be resorting to using mkdocs/mkdocs: Project documentation with Markdown. to rendering these markdown files.