[Feature Request] Add generic `markdown` output format.

This issue is **Aggregated Issues** that are relating to `markdown` output format

**Is your feature request related to a problem? Please describe.**
Some docfx users needs `markdown` output format for incorporate with external tools.
But currently docfx's `markdown` output format is intended to be used for `docfx build` command input.

And following issues are known when using `markdown` output format.
- #9879
- #9719
- #9720
- #10038

**Describe the solution you'd like**
Add new [`outputFormat`](https://dotnet.github.io/docfx/reference/docfx-json-reference.html#outputformat) option to generate markdown files that compatible with external tools.