Update actor animation tutorial

[iche033]

Updates to reflect new changes to the actor animation system in ign-rendering4

• moved Code section downward to be consistent with other tutorials
• updated steps of loading and creating the animations
• described two methods for playing the animations (manual updating bone pose and advancing animation time)

[codecov]

Codecov Report

Merging #144 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #144   +/-   ##
=======================================
  Coverage   52.07%   52.07%           
=======================================
  Files         143      143           
  Lines       13105    13105           
=======================================
  Hits         6824     6824           
  Misses       6281     6281           

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[chapulina]

👍