chore(contented-pipeline-md): restrict what directive can be parsed

[fuxingloh]

What this PR does / why we need it:

As per title—directive features should be further limited to approved components and components design or what's already defined in GFM.

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[codecov]

Codecov Report

Merging #623 (030359c) into main (0551e1b) will increase coverage by 5.36%.
Report is 62 commits behind head on main.
The diff coverage is 96.74%.

@@            Coverage Diff             @@
##             main     #623      +/-   ##
==========================================
+ Coverage   85.00%   90.36%   +5.36%     
==========================================
  Files          10       13       +3     
  Lines         420      519      +99     
  Branches       73      100      +27     
==========================================
+ Hits          357      469     +112     
+ Misses         63       50      -13     

Files	Coverage Δ
...ented-pipeline-jest-md/src/JestMarkdownPipeline.ts	100.00% <100.00%> (ø)
...ages/contented-pipeline-md/src/MarkdownPipeline.ts	85.29% <100.00%> (ø)
...ages/contented-pipeline-md/src/UnifiedProcessor.ts	87.09% <100.00%> (-0.41%)	⬇️
...contented-pipeline-md/src/plugins/RehypeHeading.ts	93.54% <ø> (+6.45%)	⬆️
...contented-pipeline-md/src/plugins/RehypeMermaid.ts	75.00% <ø> (+31.25%)	⬆️
...ntented-pipeline-md/src/plugins/RemarkCodeblock.ts	100.00% <100.00%> (ø)
...d-pipeline-md/src/plugins/RemarkDirectiveRehype.ts	100.00% <100.00%> (ø)
...es/contented-pipeline-md/src/plugins/RemarkLink.ts	100.00% <100.00%> (+28.57%)	⬆️
...ages/contented-processor/src/ContentedProcessor.ts	88.46% <100.00%> (+2.04%)	⬆️
...s/contented-pipeline-md/src/plugins/RehypeShiki.ts	96.77% <96.77%> (ø)
... and 2 more