Add/update documentation for product prefill

[sergei-maertens]

Changes

• Updated technical configuration documentation
• Split user manual/technical documentation
• Added example of how to set up a form with product prefill

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • Checked copying a form
  • Checked import/export of a form
  • Config checks in the configuration overview admin page
  • Problem detection in the admin email digest is handled

• Release management

  • I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • Ran ./bin/makemessages_js.sh
  • Ran ./bin/compilemessages_js.sh

• Dockerfile/scripts

  • Updated the Dockerfile with the necessary scripts from the ./bin folder

• Commit hygiene

  • Commit messages refer to the relevant Github issue
  • Commit messages explain the "why" of change, not the how

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.61%. Comparing base (486d9cb) to head (1e31833).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4919      +/-   ##
==========================================
- Coverage   96.62%   96.61%   -0.01%     
==========================================
  Files         760      760              
  Lines       25818    25817       -1     
  Branches     3382     3382              
==========================================
- Hits        24947    24944       -3     
- Misses        606      608       +2     
  Partials      265      265              

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[stevenbal]

Minor comments, looks good overall