Fix links on docs index page (#12750)

Author: amoeba

docs/source/index.rst

@@ -36,15 +36,15 @@ Apache DataFusion
 DataFusion is an extensible query engine written in `Rust <http://rustlang.org>`_ that
 uses `Apache Arrow <https://arrow.apache.org>`_ as its in-memory format.
 
-This documentation is for the <a href="https://github.com/apache/datafusion">core DataFusion project</a>, which contains
+This documentation is for the `core DataFusion project <https://github.com/apache/datafusion>`_, which contains
 libraries that are used to build data-centric system software. DataFusion also offers the following subprojects, which
 provide packaged versions of DataFusion intended for end users, and these have separate documentation.
 
-- <a href="https://datafusion.apache.org/python/">DataFusion Python</a> offers a Python interface for SQL and DataFrame
+- `DataFusion Python <https://datafusion.apache.org/python/>`_ offers a Python interface for SQL and DataFrame
   queries.
-- <a href="https://github.com/apache/datafusion-ray/">DataFusion Ray</a> provides a distributed version of DataFusion
-  that scales out on <a href="https://www.ray.io">Ray</a> clusters.
-- <a href="https://datafusion.apache.org/comet/">DataFusion Comet</a> is an accelerator for Apache Spark based on
+- `DataFusion Ray <https://github.com/apache/datafusion-ray/>`_ provides a distributed version of DataFusion
+  that scales out on `Ray <https://www.ray.io>`_ clusters.
+- `DataFusion Comet <https://datafusion.apache.org/comet/>`_ is an accelerator for Apache Spark based on
   DataFusion.
 
 DataFusion's target users are