Add doc for local runtime

[ryanhoangt]

• [ ] This change is worth documenting at https://docs.all-hands.dev/
• [ ] Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

End-user friendly description of the problem this fixes or functionality that this introduces.

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Give a summary of what the PR does, explaining any non-trivial design decisions.

CC: @xingyaoww

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Link of any specific issues this addresses.

Fix #6771

[mamoodi]

Just some small comments. Thanks so much for doing this!!