feat: upgrading to nextjs app router

[fern-support]

• improved color scales (grayscale and accent color scales are adjusted to more closely match the provided colors)
• more consistent styles across all markdown pages: content body is thinner for better reading experience, and border - radiuses around cards all adjusted to be uniform.
• spacing and border radius can now be adjusted using the --spacing and --radius css variables. You can now shrink the radius or make spacing around content tighter or wider to match your branding.
• better in-browser performance (less client-side javascript, and less HTML)
• bug-fixes for search—including support for mobile view
• improved sidebar (state is saved between tab navigation), and improved placement of sidebar menu in mobile view
• less content shift after navigation in most places (where this was buggy previously)
• deep linkable api explorer (by adding /~explorer at the end of urls)
• and more!

[bc-devrev]

@iarthstar can you check if this will cause any problems?

[github-actions]

Published docs preview URL: https://devrev-preview-80058882-8293-4fb6-84c3-8409612836d3.docs.buildwithfern.com