fix(docs): correct o1js navigation pagination between introduction and basic concepts pages

[wamimi]

Problem

The "Next" button on the o1js Introduction page was incorrectly:
Displaying "Introduction" as the button text instead of "Basic Concepts"
Linking back to the Introduction page instead of advancing to the Basic Concepts page
This caused a broken navigation flow for users trying to progress through the o1js documentation.

Solution
Added explicit pagination configuration to both documentation files:
docs/zkapps/o1js/index.mdx: Added pagination_next: zkapps/o1js/basic-concepts
docs/zkapps/o1js/basic-concepts.md: Added pagination_prev: zkapps/o1js/index

Result
"Next" button on Introduction page now correctly shows "Basic Concepts"
Navigation properly advances to /zkapps/o1js/basic-concepts
"Previous" button on Basic Concepts page correctly links back to Introduction
Maintains proper documentation flow through the o1js section

Testing
[x] Verified navigation works correctly in both directions
[x] Confirmed button text displays correctly
Type of change: Bug fix
Area: Documentation navigation

[vercel]

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