Add gitgraph parallel commits to docs

[NicolasCwy]

📑 Summary

Add gitgraph parallel commits to docs

Resolves #5324

📏 Design Decisions

• Released in v10.8.0 in Feature/5114 add parallel commit config #5161

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added necessary unit/e2e tests.
• 📓 have added documentation. Make sure MERMAID_RELEASE_VERSION is used for all new features.
• 🔖 targeted develop branch

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.15%. Comparing base (3093afe) to head (cbb715a).
Report is 6 commits behind head on develop.

❗ Current head cbb715a differs from pull request most recent head 370cd98. Consider uploading reports for the commit 370cd98 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5336      +/-   ##
===========================================
+ Coverage    74.89%   79.15%   +4.26%     
===========================================
  Files          156      175      +19     
  Lines        12999    14520    +1521     
  Branches       781      868      +87     
===========================================
+ Hits          9735    11494    +1759     
+ Misses        3090     2823     -267     
- Partials       174      203      +29     

Flag	Coverage Δ
e2e	84.73% <ø> (+4.63%)	⬆️
unit	43.20% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

see 51 files with indirect coverage changes