DX-1660: Use Jekyll's source directory

[asbjornu]

Use Jekyll's source directory instead of its destination directory as a base directory for PlantUML themes to simplify the integration with Jekyll a whole lot. Resolves #86.

[codecov[bot]]

Codecov Report

Merging #91 (6d11dac) into main (325cd26) will increase coverage by 1.93%. The diff coverage is 70.58%.

@@            Coverage Diff             @@
##             main      #91      +/-   ##
==========================================
+ Coverage   95.26%   97.19%   +1.93%     
==========================================
  Files          16       15       -1     
  Lines         549      464      -85     
==========================================
- Hits          523      451      -72     
+ Misses         26       13      -13     

Impacted Files	Coverage Δ
lib/kramdown-plantuml.rb	100.00% <ø> (ø)
lib/kramdown-plantuml/jekyll_provider.rb	69.56% <33.33%> (-24.06%)	:arrow_down:
lib/kramdown_html.rb	91.30% <85.71%> (-1.01%)	:arrow_down:
lib/kramdown-plantuml/plantuml_diagram.rb	100.00% <100.00%> (ø)
lib/kramdown-plantuml/theme.rb	97.91% <100.00%> (+14.58%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 325cd26...6d11dac. Read the comment docs.

[mergify[bot]]

Thank you @asbjornu for your contribution!