Add sidebar navigation to introduction page

[javiert-okta]

This PR adds a sidebar navigation to the introduction page with the following features:

• Sidebar with each heading and styling for active state:

• Clicking on a section automatically navigates to it
• Each heading now has an id so that it can be accessed via url:

https://next-jwt-8ftcynofz-okta-3f38a12f.vercel.app/introduction#how-json-web-tokens-work

• The same ids have been used for japanese version
• The design is responsive, and the sidebar will disappear if the screen is too small.

[github-actions]

Preview URL https://next-jwt-jklxajg1r-okta-3f38a12f.vercel.app

[github-actions]

Preview URL https://next-jwt-8ftcynofz-okta-3f38a12f.vercel.app

[christiansamaniego-okta]

There seems to be some issues with the first sections:

• When scrolling to the second section and then scrolling back to the first section it doesn't update the active state on the sidebar for the first section.
• When scrolling to the third section and scrolling back to the second one, it doesn't update the active state on the sidebar for the second section.

Maybe it could be fixed by using different values for threshold and rootMargin in the IntersectionObserver. 🤔

Screen.Recording.2025-08-06.at.12.59.17.PM.mov

[javiert-okta]

Looks like we can move around the value for rootMargin to fix that. Thanks @christiansamaniego-okta !

[github-actions]

Preview URL https://next-jwt-pz885xt9n-okta-3f38a12f.vercel.app

[christiansamaniego-okta]

Thank you for addressing the previous issue @javiert-okta. Looks awesome!
Nit pick: A couple very minor things I'll leave as optional:

• Make the divider match the color of the divider at the top of the page (below the "Introduction to JSON Web Tokens" heading)
• Adding some margin at the bottom so that the divider doesn't touch the component at the bottom

[github-actions]

Preview URL https://next-jwt-b68nhq5vt-okta-3f38a12f.vercel.app

[javiert-okta]

Nice suggestions @christiansamaniego-okta. I addressed both of them, take a look.

[christiansamaniego-okta]

LGTM! Good job!