Support documents in _posts

[durd07]

Fixes #84.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #85 (7f15c53) into main (5b73a67) will decrease coverage by 0.51%.
The diff coverage is 25.00%.

❗ Current head 7f15c53 differs from pull request most recent head 17573cb. Consider uploading reports for the commit 17573cb to get more accurate results

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
- Coverage   95.77%   95.26%   -0.52%     
==========================================
  Files          16       16              
  Lines         545      549       +4     
==========================================
+ Hits          522      523       +1     
- Misses         23       26       +3     

Impacted Files	Coverage Δ
lib/kramdown-plantuml/jekyll_provider.rb	93.61% <25.00%> (-6.39%)	⬇️

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[mergify]

Thank you @durd07 for your contribution!