ci(enhancement): Do not trigger Github Actions for doc or Readme changes

[coder3101]

As pointed out by @jwakely in this comment #99 (comment) it is not wise to trigger CI for changes in docs or any other non-code related changes. This commit relaxes the trigger restriction on PR and Push event so that they are only triggered when at least one file changed which does not belong to the below list:

• Markdown changed (anything ending with .md)
• Anything inside doc subdirectory.

[coder3101]

Travis and Appveyor will still trigger. They are legacy CI we will drop in future, so let them get triggered until their removal.

[codecov]

Codecov Report

Merging #102 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #102   +/-   ##
========================================
  Coverage    95.33%   95.33%           
========================================
  Files           22       22           
  Lines         1223     1223           
========================================
  Hits          1166     1166           
  Misses          57       57           

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[bassoy]

@coder3101
I think it would make sense to have a Github-Action for the CMAKE on a windows and linux machine.

[coder3101]

Okay, I will add it after my CMake PR is merged.