docs: fix README rendering and various issues

[regisb]

GitHub could not render the README file because of issues with titles. We take the opportunity to solve a few additional issues.

[openedx-webhooks]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (70cc2d8) 70.22% compared to head (c9b1b96) 70.22%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #227   +/-   ##
=======================================
  Coverage   70.22%   70.22%           
=======================================
  Files          27       27           
  Lines         403      403           
  Branches       85       85           
=======================================
  Hits          283      283           
  Misses        119      119           
  Partials        1        1           

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[openedx-webhooks]

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