[dev docs] GitHub Workflow: simplify & convert to vanilla markdown

[lukeelmers]

Summary

• Migrate GitHub Workflow documentation from .mdx to .md format to prepare for migration away from docsmobile
• Move content from dev_docs/contributing/how_we_use_github.mdx to dev_docs/migrated_docs/how_we_use_github.md
• Rewrite content (with the help of AI) for clarity and brevity

[elasticmachine]

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