docs: update README

[minaelee]

• Added Requirements section
• Had user name new repo 'fern-docs' instead of saying "something like", to simplify the experience by having them make less decisions
• Removed fern check since not needed for this QS as we supply the OpenAPI spec file, again to simplify the experience
• Removed step on using your own OpenAPI specification file. fern init --openapi command will fail if you run it the way that step suggests, with error message that openapi/ folder already exists. Also, the step on customizing tells users how to use their own OpenAPI file.
• Edited step on customizing to provide more details and direct users to Writing Content page for information on how to change docs.yml for navigation and custom styles.

[dannysheridan]

nice clarity on the Advanced options!