docs: document ELK layout registration for flowcharts

[AgnetaGavrielov]

Summary

• documents how to install and register @mermaid-js/layout-elk when using Mermaid outside the Live Editor
• adds bundler and CDN examples using mermaid.registerLayoutLoaders
• clarifies that ELK can then be selected with flowchart.defaultRenderer or layout: elk

Fixes #5969

Checklist

• Documentation only
• Tests are not needed for this documentation-only change

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commit: 8117f72

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 24.36%. Comparing base (6a6a562) to head (8117f72).

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #7900       +/-   ##
============================================
- Coverage    67.79%   24.36%   -43.44%     
============================================
  Files          429       30      -399     
  Lines        38648     6925    -31723     
  Branches      6364      337     -6027     
============================================
- Hits         26201     1687    -24514     
+ Misses        9675     5238     -4437     
+ Partials      2772        0     -2772     

Flag	Coverage Δ
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unit	24.36% <ø> (ø)

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