feat: experimental support for markdown output

[yufeih]

Allow generating markdown output with docfx metadata --outputFormat markdown or { "outputFormat": "markdown" } JSON config.

• Supports basic markdown output for class, struct, interface, delegate and enums, TOC.
• Does not support namespace pages.
• Does not support proper markdown escaping.
• Does not support in-page anchors.
• Does not support separated member pages and nested namespace.
• Does not support merging extension methods
• Does not support localized markdown pages.

Design Note:

• Markdown support is added to metadata command so it will not work with build command features such as overwrite files and custom template.
• Markdown support is built directly on Roslyn, as opposed to the ManagedReference YAML model. This may introduce subtle behavior differences with the HTML output. Overtime, we expect to converge both formats to sit directly on Roslyn and address behavior gaps.

#684

[codecov]

Codecov Report

Patch coverage is 4.24% of modified lines.

Files Changed	Coverage
src/Docfx.Dotnet/MarkdownFormatter.cs	0.00%
src/Docfx.Dotnet/SymbolHelper.cs	0.00%
...fx.Dotnet/ExtractMetadata/ExtractMetadataWorker.cs	33.33%
src/Docfx.Dotnet/SymbolUrlResolver.cs	88.88%
src/Docfx.Dotnet/DotnetApiCatalog.cs	100.00%
...fx.Dotnet/ExtractMetadata/ExtractMetadataConfig.cs	100.00%
src/Docfx.Dotnet/MetadataJsonConfig.cs	100.00%
src/Docfx.Dotnet/SymbolFormatter.cs	100.00%
src/docfx/Models/MetadataCommand.cs	100.00%
src/docfx/Models/MetadataCommandOptions.cs	100.00%

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