docs: 📝 add docs to custom render method

[dennemark]

Add documentation to custom render method - Discussion: maplibre#3136 (comment)

Launch Checklist

• Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• Briefly describe the changes in this PR.
• Link to related issues.
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality.
• Document any changes to public APIs.
• Post benchmark scores.
• Add an entry to CHANGELOG.md under the ## main section.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.57%. Comparing base (4046e53) to head (20af33f).
Report is 1 commits behind head on expose-projection-matrix-params.

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Additional details and impacted files

@@                         Coverage Diff                         @@
##           expose-projection-matrix-params     #209      +/-   ##
===================================================================
- Coverage                            87.68%   87.57%   -0.11%     
===================================================================
  Files                                  242      242              
  Lines                                33085    33092       +7     
  Branches                              2171     2314     +143     
===================================================================
- Hits                                 29009    28981      -28     
- Misses                                3083     3117      +34     
- Partials                               993      994       +1     

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[dennemark]

@birkskyum did you find time to look at this already? i think afterwards the PR on malibre repo is nearly ready to go. not sure about tests though...