Add API document generation target projects.

[filzrev]

This PR including following changes.

• Include all .csproj as extract metadata targets. (To reference contents by xref:)
  And fix related warnings occurred when running docfx metadata command.
• Remove README.md file from docs directory.
  Because it' contents seems obsolete. and published to site

List of rarget projects to extract API metadata

docfx.csproj
Docfx.App.csproj
Docfx.Build.Common.csproj
Docfx.Build.ConceptualDocuments.csproj
Docfx.Build.Engine.csproj
Docfx.Build.ManagedReference.csproj
Docfx.Build.OverwriteDocuments.csproj
Docfx.Build.ResourceFiles.csproj
Docfx.Build.RestApi.csproj
Docfx.Build.SchemaDriven.csproj
Docfx.Build.TableOfContents.csproj
Docfx.Build.UniversalReference.csproj
Docfx.Common.csproj
Docfx.DataContracts.Common.csproj
Docfx.DataContracts.RestApi.csproj
Docfx.DataContracts.UniversalReference.csproj
Docfx.Dotnet.csproj
Docfx.Glob.csproj
Docfx.HtmlToPdf.csproj
Docfx.MarkdigEngine.csproj
Docfx.MarkdigEngine.Extensions.csproj
Docfx.MarkdigEngine.Validators.csproj
Docfx.Plugins.csproj
Docfx.YamlSerialization.csproj

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.01% ⚠️

Comparison is base (dd59353) 76.88% compared to head (f02cd98) 76.88%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8945      +/-   ##
==========================================
- Coverage   76.88%   76.88%   -0.01%     
==========================================
  Files         591      591              
  Lines       24758    24758              
==========================================
- Hits        19036    19035       -1     
- Misses       5722     5723       +1     

Files Changed	Coverage Δ
...digEngine.Extensions/Inclusion/InclusionContext.cs	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

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[yufeih]

As mentioned in #8944, many internal APIs are exposed as public, and many are missing documentation. We may want to review, document and light up these APIs piece by piece.

As a next step, we may want to start with internalizing APIs that are unintentionally made public, and identify, document and expose public APIs that we want to support for the long term.

[filzrev]

Rebase branch to latest main

[filzrev]

Close this PR and recreate PR that includes

• Remove README.md
• Fix Doc comments