fix badge link

[lmiq]

This PR aims to fix the issue with the badge link that is currently provided. Fixes #353

Apparently github changed the policy regarding access to raw files, thus it is better to link an image from the documentation. Here I add the badge info to the documentation and the badge as an asset of of the docs, which then provides a link that currently works.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.77%. Comparing base (62eda50) to head (0a2251c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #354   +/-   ##
=======================================
  Coverage   86.77%   86.77%           
=======================================
  Files          11       11           
  Lines         514      514           
=======================================
  Hits          446      446           
  Misses         68       68           

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[lmiq]

I think this is a good strategy: the link refers to the dev docs badge. Since the badge is referenced in the docs, if for some reason the file is removed or is not accessible, the docs won't build, and thus we guarantee that the badge is always accessible.