feat: memberpage plugin -> memberLayout.SeparatePages

[yufeih]

Add memberLayout option to enable generating type members as separate pages using:

{
   "metadata": [{
      "memberLayout": "separatePages"
   }]
}

Fixes #8389

[codecov]

Codecov Report

Patch coverage: 96.29% and project coverage change: -0.97 ⚠️

Comparison is base (347fc98) 75.30% compared to head (a9c1a8a) 74.34%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8569      +/-   ##
==========================================
- Coverage   75.30%   74.34%   -0.97%     
==========================================
  Files         649      648       -1     
  Lines       23926    23935       +9     
==========================================
- Hits        18018    17795     -223     
- Misses       5908     6140     +232     

Impacted Files	Coverage Δ
...c/Microsoft.DocAsCode.Dotnet/MetadataJsonConfig.cs	94.73% <93.75%> (-5.27%)	⬇️
...ld.ManagedReference/SplitClassPageToMemberLevel.cs	9.66% <100.00%> (ø)
src/Microsoft.DocAsCode.Dotnet/DotnetApiCatalog.cs	72.41% <100.00%> (+0.48%)	⬆️
...de.Dotnet/ExtractMetadata/ExtractMetadataConfig.cs	100.00% <100.00%> (ø)
...de.Dotnet/ExtractMetadata/ExtractMetadataWorker.cs	76.03% <100.00%> (+0.56%)	⬆️
...DocAsCode.Dotnet/ManagedReference/PageViewModel.cs	100.00% <100.00%> (ø)

... and 9 files with indirect coverage changes

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