[Feature Request] Add generic markdown output format.

[filzrev]

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.

• [Bug] .NET API / Markdown: Unresolved cref / xref tags from <summary> in generated markdown. #9879
• [Bug] < in header not escaped when outputFormat: "markdown" #9719
• [Feature Request] Make <a/> tag in headers optional when outputFormat: markdown #9720
• Markdown as the end result with nesting #10038

Describe the solution you'd like
Add new outputFormat option to generate markdown files that compatible with external tools.