add 'readthedocs' config file

[danieleades]

closes #29

[codecov]

Codecov Report

Merging #28 (71df87c) into main (0f077a6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 71df87c differs from pull request most recent head f9e82ed. Consider uploading reports for the commit f9e82ed to get more accurate results

@@           Coverage Diff           @@
##             main      #28   +/-   ##
=======================================
  Coverage   98.30%   98.31%           
=======================================
  Files          11       11           
  Lines         295      296    +1     
=======================================
+ Hits          290      291    +1     
  Misses          5        5           

Impacted Files	Coverage Δ
src/sphinx_graph/directives/vertex/directive.py	100.00% <100.00%> (ø)
src/sphinx_graph/directives/vertex/events.py	94.73% <100.00%> (+0.14%)	⬆️

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