Update gitlab_flow.md

What does this MR do and why?

Describe in detail what your merge request does and why.

Updated the GitLab Flow page page. Removed or deleted references to competitors. Added more text related to how things GitLab Flow does things.

Related links

• Related to Gitlab Flow Docs - Adding Enablement Content fo... (#378297 - closed)
• Related to Update and Validate GitLab Flow documentation (#333160 - closed)

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MR acceptance checklist

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