Fix broken link and missing nav page reference

[mintlify]

Summary

Two issues found and fixed:

1. Broken link in data-infrastructure/big-query.mdx

• Link: ../getting-started/basics
• Problem: The page getting-started/basics does not exist. Also used a relative path (../) instead of a root-relative path.
• Fix: Changed to /getting-started/what-is-near, which is the current equivalent overview page for the NEAR protocol. Also converted the adjacent link (Near Data flow) to use a root-relative path.

2. Missing nav entry in docs.json

• Reference: protocol/transactions (line 132)
• Problem: protocol/transactions.mdx does not exist. The content lives in protocol/transactions/index.mdx, but Mintlify's validate tool does not resolve the bare folder path to the index file.
• Fix: Changed nav entry from protocol/transactions to protocol/transactions/index.

Validation

Both mint validate and mint broken-links pass after these changes.