feat: New extensibility model based on libraries

[yufeih]

This is a new extensibility model by shipping docfx as a library. It changes the extensibility model from runtime to build time:

• Microsoft.DocAsCode is a new library package with public APIs to build a project with the exact same behavior as executing the docfx CLI.
• The docfx package is a thin command line wrapper around Microsoft.DocAsCode.
• To enable extensibility, create a new EXE project and add Microsoft.DocAsCode package
• It should also work seamlessly with more script-oriented build systems like dotnet-script or cake.
• Extensions like memberpage are shipped as ordinary NuGet libraries, to use an extension, add it to the project using <PackageReference>.
• More extensibility options like markdown extensions, template extensions can be exposed through Microsoft.DocAsCode with additional APIs for customization.

See samples/extensions folder on an EXE project sample.

This approach solves the problems of sharing+consuming extensions and assembly loading issues with the plugin approach.
Loading plugins dynamically was removed with this change.

NOTE: This PR makes memberpage C# code work, but the memberpage templates are not yet solved.

The initial APIs exposed by this PR are:

namespace Microsoft.DocAsCode;

public class Docset
{
    public static Task Build(string configPath);
}

cc @vicancy

[vicancy]

Looks awesome! Some little comments on the API surface

[codecov]

Codecov Report

Base: 72.79% // Head: 73.25% // Increases project coverage by +0.46% 🎉

Coverage data is based on head (fde1265) compared to base (d828308).
Patch coverage: 26.12% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8306      +/-   ##
==========================================
+ Coverage   72.79%   73.25%   +0.46%     
==========================================
  Files         953      955       +2     
  Lines       38680    38627      -53     
  Branches     7191     7186       -5     
==========================================
+ Hits        28158    28298     +140     
+ Misses       8387     8204     -183     
+ Partials     2135     2125      -10     

Impacted Files	Coverage Δ
src/Microsoft.DocAsCode/Config/BuildJsonConfig.cs	90.19% <ø> (ø)
...c/Microsoft.DocAsCode/Config/ContentPairingInfo.cs	0.00% <ø> (ø)
src/Microsoft.DocAsCode/Config/FileItems.cs	71.42% <ø> (ø)
src/Microsoft.DocAsCode/Config/FileMapping.cs	73.33% <ø> (ø)
...Microsoft.DocAsCode/Config/FileMappingConverter.cs	83.33% <ø> (ø)
src/Microsoft.DocAsCode/Config/FileMappingItem.cs	82.75% <ø> (ø)
...rc/Microsoft.DocAsCode/Config/FileMetadataPairs.cs	66.66% <ø> (ø)
...oft.DocAsCode/Config/FileMetadataPairsConverter.cs	72.22% <ø> (ø)
...icrosoft.DocAsCode/Config/FileMetadataPairsItem.cs	100.00% <ø> (ø)
src/Microsoft.DocAsCode/Config/FileModelParser.cs	20.00% <ø> (ø)
... and 43 more

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