Add redirection document type

[filzrev]

Add new DocumentType Redirection according to comments on #8821.

What's included in this PR

1. Add Redirection document type to DocumentType.
2. Add logics to change document type from "Conceptual" to Redirection. If redirect_url metadata is specified.
3. Add Redirection.html.primary.tmpl to default build-in template.
   -> Redirection documents are processed this Mustache template only. (Don't use _master.tmpl)
4. Modify SitemapGenerator.cs to skip Redirection type documents. and some code refactoring.
5. Test code for redirect_url .

Note
_master.tmpl's conditional statement ({{#redirect_url}}...{{/redirect_url}}) are left as is.
Because redirect_url can be set via global.json and may effects other DocumentTypes.

[codecov]

Codecov Report

Patch coverage: 87.50% and project coverage change: +0.04 🎉

Comparison is base (12f3998) 77.00% compared to head (6703ad6) 77.04%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8892      +/-   ##
==========================================
+ Coverage   77.00%   77.04%   +0.04%     
==========================================
  Files         605      605              
  Lines       25036    25040       +4     
==========================================
+ Hits        19278    19292      +14     
+ Misses       5758     5748      -10     

Impacted Files	Coverage Δ
...de.Build.Engine/PostProcessors/SitemapGenerator.cs	83.33% <82.35%> (+14.21%)	⬆️
...ConceptualDocuments/ConceptualDocumentProcessor.cs	63.52% <100.00%> (+2.76%)	⬆️

... and 2 files with indirect coverage changes

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[yufeih]

Verify that:

• the generated sitemap does not contain redirections
• the content type in the generated manifest file is redirection

[filzrev]

Fix DocumentType constant definitions in this commit (17fa984)
And unit tests are added related to redirect_url and sitemap generation. (a09b8f8)

[yufeih]

This is awesome, really appreciate all the hard work you put into this! ❤️❤️❤️