Add documentation for Git Graph parallel commits

[NicolasCwy]

Description

Realised that there is a parallel commit configuration option for Git Graph while going through the code base but couldn't find anything about it in the docs

Steps to reproduce

1. Go to https://mermaid.js.org/syntax/gitgraph.html
2. Search for "parallel commits"

Screenshots

Parallel commits enabled
mermaid.initialize({ startOnLoad: true, theme: "dark", gitGraph: { parallelCommits: true }, });

Code Sample

No response

Setup

• Mermaid version: Added in v10.8.0

Suggested Solutions

No response

Additional Context

No response

[NicolasCwy]

Will be working on this!