feat: improvements over Pluto conversions

[storopoli]

• convert admonitions
• add let ... end blocks
• format: indent = 4
• add more frontmatter stuff

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ce0daac) 84.35% compared to head (7ff8e2e) 84.35%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #10   +/-   ##
=======================================
  Coverage   84.35%   84.35%           
=======================================
  Files           5        5           
  Lines         358      358           
=======================================
  Hits          302      302           
  Misses         56       56           

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