Fix the everest doc generator

[verveerpj]

The generator would only function within a
editable install. The behavior is changed such
that it has to be run from within the ERT
repository, in one of the parent directories of
the file that is generated.

Issue
Resolves #8821

Approach
Change the generator to function only when started within the ERT source tree.

(Screenshot of new behavior in GUI if applicable)

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.38%. Comparing base (e607feb) to head (967abff).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8822      +/-   ##
==========================================
+ Coverage   91.31%   91.38%   +0.06%     
==========================================
  Files         342      342              
  Lines       21028    21028              
==========================================
+ Hits        19202    19216      +14     
+ Misses       1826     1812      -14     

Flag	Coverage Δ
cli-tests	39.59% <ø> (-0.05%)	⬇️
gui-tests	73.58% <ø> (+0.12%)	⬆️
performance-tests	50.05% <ø> (ø)
unit-tests	79.82% <ø> (-0.02%)	⬇️

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