Improve quickstart setup experience

We would like to improve the experience of running chat-ui locally and reduce friction in the setup.

Current pain points:

• Having to setup your DB
• Having to point to an inference server
• Having to deal with either docker or git + npm

The goal is to reduce the friction with the following steps:

• Make the DB optional, use an in-process mongo server if it's not set (#1773)
• Make it possible to download and run models directly from chat-ui (with node-llama-cpp?) #1778
• Allow admin login from CLI à la jupyter notebook (#1789 )
• Add an onboarding UI for setting up new models from the app
• Provide an easy setup like npx @huggingface/chat-ui or pip
• Update the README so it focuses on quickstarting and move the rest of the doc to the docs page