feat: allow custom alert types

[yufeih]

Allow custom alerts with the build.markdownEngineProperties.alerts property in docfx.json. The key specifies the markdown keyword without the surrounding [!, ] symbols. The value is the CSS class names.

Alert titles are by default the alert keyword in upper case and can be overridden in token.json for localization and other purposes.

We choose this approach over the >[!TIP](a title) syntax because:

• DRY: it avoids the repetition of titles in markdown
• Opens room for adding new alert types
• It is easier to localize titles

fixes #7939
fixes #8391
fixes #6268

[codecov]

Codecov Report

Patch coverage: 90.00% and project coverage change: +0.05 🎉

Comparison is base (2fe10f3) 76.91% compared to head (85ce2a2) 76.96%.

❗ Current head 85ce2a2 differs from pull request most recent head a202fe0. Consider uploading reports for the commit a202fe0 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8735      +/-   ##
==========================================
+ Coverage   76.91%   76.96%   +0.05%     
==========================================
  Files         606      606              
  Lines       25065    25062       -3     
==========================================
+ Hits        19278    19290      +12     
+ Misses       5787     5772      -15     

Impacted Files	Coverage Δ
...ft.DocAsCode.App/Helpers/DocumentBuilderWrapper.cs	71.83% <0.00%> (ø)
...ft.DocAsCode.MarkdigEngine.Extensions/Constants.cs	100.00% <ø> (ø)
src/docfx/Models/BuildCommand.cs	45.45% <0.00%> (+7.71%)	⬆️
....DocAsCode.MarkdigEngine/MarkdigMarkdownService.cs	87.36% <75.00%> (-0.52%)	⬇️
...ions/QuoteSectionNote/QuoteSectionNoteExtension.cs	81.81% <81.25%> (ø)
...ine/ResourceFileReaders/CompositeResourceReader.cs	88.88% <90.90%> (-6.95%)	⬇️
.../Microsoft.DocAsCode.App/Config/BuildJsonConfig.cs	86.11% <100.00%> (ø)
...oft.DocAsCode.Build.Engine/DocumentBuildContext.cs	66.66% <100.00%> (+0.54%)	⬆️
....DocAsCode.Build.Engine/DocumentBuildParameters.cs	100.00% <100.00%> (ø)
...Build.Engine/TemplateProcessors/TemplateManager.cs	88.63% <100.00%> (-0.26%)	⬇️
... and 5 more

... and 3 files with indirect coverage changes

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[KalleOlaviNiemitalo]

The use of token.json in templates doesn't seem to be documented.