DX-914: Switch from `post_render` to `post_write`

[asbjornu]

Switch from :site:post_render to :site:post_write so the PlantUML theme can be written to disk by Jekyll before PlantUML starts looking for it. Also fix a few other minor things.

[codecov[bot]]

Codecov Report

Merging #74 (1e56be0) into main (1db4250) will decrease coverage by 1.59%. The diff coverage is 78.04%.

@@            Coverage Diff             @@
##             main      #74      +/-   ##
==========================================
- Coverage   95.62%   94.03%   -1.60%     
==========================================
  Files          12       12              
  Lines         389      419      +30     
==========================================
+ Hits          372      394      +22     
- Misses         17       25       +8     

Impacted Files	Coverage Δ
lib/kramdown_html.rb	92.30% <0.00%> (-3.70%)	:arrow_down:
lib/kramdown-plantuml/theme.rb	81.81% <50.00%> (-14.96%)	:arrow_down:
lib/kramdown-plantuml/jekyll_provider.rb	86.30% <96.15%> (+3.84%)	:arrow_up:

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[mergify[bot]]

Thank you @asbjornu for your contribution!