Doctests and documentation patch

[abhro]

Main changes:

• Add more doctests
• Add syntax highlighting tags for code examples
• Remove unnecessary naming from doctests that cause them to share scope (no variables are shared, scope sharing isn't necessary)
• Pull items into lists

[codecov]

Codecov Report

Attention: Patch coverage is 37.50000% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 75.69%. Comparing base (87f02db) to head (67a3ad9).

Files	Patch %	Lines
...c/rational-functions/rational-transfer-function.jl	0.00%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #571   +/-   ##
=======================================
  Coverage   75.69%   75.69%           
=======================================
  Files          38       38           
  Lines        4037     4037           
=======================================
  Hits         3056     3056           
  Misses        981      981           

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[jverzani]

Thanks so much for this! Apologies for taking so long to merge, I had missed the notification.