Remove webviz-ert documentation

[andreas-el]

Remove some leftovers after #12567

• 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 'just rapid-tests')

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: Add backport label to latest release (format: 'backport release-branch-name')

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.64%. Comparing base (e0162ab) to head (21cde52).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12649      +/-   ##
==========================================
- Coverage   90.68%   90.64%   -0.05%     
==========================================
  Files         429      429              
  Lines       29804    29804              
==========================================
- Hits        27028    27016      -12     
- Misses       2776     2788      +12     

Flag	Coverage Δ
cli-tests	37.56% <ø> (ø)
gui-tests	69.37% <ø> (-0.03%)	⬇️
performance-and-unit-tests	73.91% <ø> (-0.03%)	⬇️
test	38.07% <ø> (-0.01%)	⬇️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 22 untouched benchmarks

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_{Comparing andreas-el:remove_webviz_ert_docs (21cde52) with main (e0162ab)}