fix: ApiPage is not generated when multiple namespaces defined across projects

[filzrev]

This PR is intended to fix #9618.

On current implementation.
CreateNamespaceToc returns only first namespace that is processed.

This PR removing idExists check for CreateNamespaceToc.
It's because Namespace with same id can be defined by multiple projects.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (e2f78e9) 74.31% compared to head (8c91a60) 74.32%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/Docfx.Dotnet/DotnetApiCatalog.Toc.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9647   +/-   ##
=======================================
  Coverage   74.31%   74.32%           
=======================================
  Files         536      536           
  Lines       23179    23177    -2     
  Branches     4054     4054           
=======================================
  Hits        17226    17226           
+ Misses       4851     4849    -2     
  Partials     1102     1102           

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[filzrev]

It can generate .md files successfully by this PR.
But after running docfx build command. TOC entries are duplicated.
It's needs to implement tock merge inside CreateNamespaceToc method.