docs: fix broken SQL & DataFrame links in root README (#18153) (#18274)

## Which issue does this PR close?
- Closes #18153

## Rationale for this change
The SQL and DataFrame documentation links in the root README were broken
and returned 404 errors.
This PR updates the URLs to point to the correct documentation paths,
improving navigation for new users and contributors.

## Are these changes tested?
Verified that the new links are valid and accessible.

Author: manasa-manoj-nbr

README.md

@@ -64,7 +64,7 @@ See [use cases] for examples. The following related subprojects target end users
   DataFusion.
 
 "Out of the box,"
-DataFusion offers [SQL] and [`Dataframe`] APIs, excellent [performance],
+DataFusion offers [SQL](https://datafusion.apache.org/user-guide/sql/index.html) and [Dataframe](https://datafusion.apache.org/user-guide/dataframe.html) APIs, excellent [performance],
 built-in support for CSV, Parquet, JSON, and Avro, extensive customization, and
 a great community.