Docs: API pages, `Makefile` fix

[tony]

• Split /api.md into multiple pages / files /api/
• Fixes for Makefile w/ make start in docs/

[codecov[bot]]

Codecov Report

Merging #283 (11b8701) into master (73fae86) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #283   +/-   ##
=======================================
  Coverage   93.30%   93.30%           
=======================================
  Files          14       14           
  Lines        1045     1045           
  Branches      175      175           
=======================================
  Hits          975      975           
  Misses         51       51           
  Partials       19       19           

Impacted Files	Coverage Δ
src/unihan_etl/util.py	55.55% <ø> (ø)
src/unihan_etl/types.py	93.54% <100.00%> (ø)

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