📝 [open-zaak/open-zaak#1649] Document envvars

[stevenbal]

Fixes https://github.com/open-zaak/open-zaak/issues/1649 partially

Changes

• Document the envvars
• Add CI job to check if envvar docs were updated

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.79%. Comparing base (ba752ea) to head (cefabbf). Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #206 +/- ## ======================================= Coverage 96.79% 96.79% ======================================= Files 143 143 Lines 6272 6272 ======================================= Hits 6071 6071 Misses 201 201 ```

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