Added guidance for custom widgets

[smithdc1]

We see a number of issues where folk are trying to use custom widgets in their form (#768, #971 etc). Given the myriad of things these could be I'm not sure there's a simple generic answer that could work.

I think the best we can do is to guide folk to use a custom template, as this is feeling quite bespoke at this point?

Closes #768
Closes #971

[codecov-commenter]

Codecov Report

Merging #1158 (1edab21) into main (f5185a1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1158   +/-   ##
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  Coverage   97.24%   97.24%           
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  Files          23       23           
  Lines        2906     2906           
  Branches      243      243           
=======================================
  Hits         2826     2826           
  Misses         39       39           
  Partials       41       41           

Flag	Coverage Δ
unittests	97.24% <ø> (ø)

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[carltongibson]

Yep. 😃

A widget is at least 50% the actual HTML it generates. I think this is fair

[smithdc1]

Thank you for the review! 👍