docs: small fixes based on friction log

[shahednasser]

• Changed Set up dev environment description
• Changed usage of "Headless Backend" with "Medusa Server" in Introduction page
• Renamed the Choose Medusa Starter heading in create-medusa-app
• Remove python from the Test Server command in quickstart guides
• Added a note about stored passwords

Closes DOCS-410, DOCS-411, DOCS-412, DOCS-413, DOCS-414, and DOCS-415

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