DOCS: Minor documentation improvement

[Miksus]

This PR includes the following docs changes:

• New code highlight colors
• Section about observer tasks (permanent option)
• Improved task termination section
• Improved contributions guide

[codecov-commenter]

Codecov Report

Base: 94.88% // Head: 94.90% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (c63541e) compared to base (2348c90).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
+ Coverage   94.88%   94.90%   +0.02%     
==========================================
  Files          80       80              
  Lines        4454     4454              
==========================================
+ Hits         4226     4227       +1     
+ Misses        228      227       -1     

Impacted Files	Coverage Δ
rocketry/core/schedule.py	93.50% <0.00%> (+0.36%)	⬆️

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