Updates to tutorials

[chapulina]

• Instruct user to clone this repository: users landing in https://ignitionrobotics.org/api/rendering/4.0/tutorials.html don't know where to start from otherwise
• Split command from command results, so it's clearer to the user what they need to do
• Fix the custom shaders tutorial - it was telling the user to compile the camera tracking

I updated many tutorials, but I only walked through these ones:

• custom shaders
• particles
• text geometry
• render pass

There may be more things to fix on the other ones.

[codecov]

Codecov Report

Merging #143 into master will not change coverage.
The diff coverage is n/a.

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

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[iche033]

looks good to me