Update heat equation doc with DBL setup

[xjules]

Issue
Update docs to heat equation with DBL setup.

• add images to show the difference between GLOBAL, ADAPTIVE and DISTANCE localization
  Approach
  📓

(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 'just rapid-tests')

When applicable

• When screenshots are changed: Review screenshot-PR in ert-testdata,
  merge screenshot-PR in ert-testdata before merging this PR.
• 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 91.75%. Comparing base (4e36dd4) to head (18f2a47).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14331      +/-   ##
==========================================
- Coverage   91.75%   91.75%   -0.01%     
==========================================
  Files         485      485              
  Lines       34708    34708              
==========================================
- Hits        31848    31846       -2     
- Misses       2860     2862       +2     

Flag	Coverage Δ
cli-tests	35.70% <ø> (ø)
fuzz	43.89% <ø> (ø)
gui-tests	58.17% <ø> (-0.01%)	⬇️
performance-and-unit-tests	80.97% <ø> (-0.09%)	⬇️
test	45.29% <ø> (+<0.01%)	⬆️

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see 2 files with indirect coverage changes

[codspeed-hq]

Merging this PR will not alter performance

✅ 36 untouched benchmarks

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_{Comparing xjules:doc_heat_eq (18f2a47) with main (4e36dd4)}