Update eherkenning docs

[sergei-maertens]

Partly closes #4785

The documentation update is independent from the library changes, but it would be great if both are done in one go.

Changes

• Removed the warnings that configuration in the admin is not possible
• Added section about CSR generation
• Updated configuration steps with moved/new fields from django-digid-eherkenning library fields

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 96.62%. Comparing base (0e095fd) to head (6761812).
Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4960   +/-   ##
=======================================
  Coverage   96.62%   96.62%           
=======================================
  Files         760      760           
  Lines       25838    25838           
  Branches     3386     3386           
=======================================
  Hits        24965    24965           
  Misses        608      608           
  Partials      265      265           

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