Update conceptual info about docs.json

[ethanpalm]

Documentation changes

https://mintlify.com/docs/settings jumps into reference information without explaining what a docs.json file does and why you need one.

This PR builds out the conceptual information on docs.json and why it's pivotal to your docs site.

Changes include

• Add intro content on what the file does
• Add setting up section that has the minimum required parameters to function
• Rename to "Global settings" to avoid any confusion with user or account settings
• Add examples of a typical docs.json for a SaaS company, with API playground, and with multiple languages
• Revise all reference content for clarity and completeness
• Include a link to settings as a call to action at the end of the quickstart

Closes https://linear.app/mintlify/issue/DOC-57/add-conceptual-information-to-settings
Closes https://linear.app/mintlify/issue/DOC-54/docsjson-page-in-search-results

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For Reviewers

When reviewing documentation PRs, please consider:

✅ Technical accuracy

• Code examples work as written
• Commands and configurations are correct
• Links resolve to the right destinations
• Prerequisites and requirements are accurate

✅ Clarity and completeness

• Instructions are clear and easy to follow
• Steps are in logical order
• Nothing important is missing
• Examples help illustrate the concepts

✅ User experience

• A new user could follow these docs successfully
• Common gotchas or edge cases are addressed
• Error messages or troubleshooting guidance is helpful