[AAE-3493] Update the documentation to provide the proper guidance on custom forms widgets for APA and APS developers

[dhrn]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation
• Other... Please describe:

What is the current behaviour? (You can also link to an open issue here)

What is the new behaviour?

Does this PR introduce a breaking change? (check one with "x")

• Yes
• No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[m-hulbert]

Let me know if I've understood the functionality correctly and this still makes sense and I can take a look at the APS stencil section too.

[m-hulbert]

LGTM 👍

[codecov-commenter]

Codecov Report

Merging #6158 into develop will increase coverage by 1.30%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #6158      +/-   ##
===========================================
+ Coverage    76.31%   77.61%   +1.30%     
===========================================
  Files          850      144     -706     
  Lines        18981     4025   -14956     
  Branches      3753      799    -2954     
===========================================
- Hits         14485     3124   -11361     
+ Misses        3360      643    -2717     
+ Partials      1136      258     -878     

Impacted Files	Coverage Δ
...ocument-list/components/document-list.component.ts	83.53% <0.00%> (-0.78%)	⬇️
...ices/src/lib/document-list/document-list.module.ts	100.00% <0.00%> (ø)
...t/components/process-instance-details.component.ts
lib/core/services/get-type.ts
...ces/src/lib/task-list/models/task-details.event.ts
.../lib/process-list/models/process-instance.model.ts
...src/lib/task/directives/complete-task.directive.ts
...ib/task/directives/unclaim-task-cloud.directive.ts
lib/core/layout/layout.module.ts
...ss-list/components/process-list-cloud.component.ts
... and 704 more

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