Document parallel execution configuration and cross-browser testing (#368)

* Initial plan

* Add comprehensive parallel execution documentation

Co-authored-by: MohabMohie <19201898+MohabMohie@users.noreply.github.com>

* Fix typo in parallel execution demo repository URL

Co-authored-by: MohabMohie <19201898+MohabMohie@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: MohabMohie <19201898+MohabMohie@users.noreply.github.com>

Author: Copilot