docs: replacement steps from apps with custom functions

[lukegalbraithrussell]

Summary

Steps from maps is deprecated. Custom functions is an alternative.

• Adds custom functions docs. Duplicative with api.slack.com, but shorter and serves a slightly different purpose methinks. Will reassess
• Rewrote deprecation warning and fixed CSS

ToC

Corrected Deprecation bubble

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've once again asked @zimeg for help with troubleshooting

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.00%. Comparing base (044e3c5) to head (c4a8688).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2149   +/-   ##
=======================================
  Coverage   82.00%   82.00%           
=======================================
  Files          18       18           
  Lines        1539     1539           
  Branches      442      442           
=======================================
  Hits         1262     1262           
  Misses        179      179           
  Partials       98       98           

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[lukegalbraithrussell]

these weren't tutorials so weren't showing up here.

[filmaj]

Left a few suggestions / questions but non-blocking.