Update _index.md

Add button for matnwb documentation

Author: ehennestad

content/converting-data-to-nwb/_index.md

@@ -73,6 +73,8 @@ available_tools_section:
           resources:
             - text: "GitHub Repository"
               url: "https://github.com/NeurodataWithoutBorders/matnwb"
+            - text: "Documentation"
+              url: "https://matnwb.readthedocs.io"
             - text: "Tutorials"
               url: "https://github.com/NeurodataWithoutBorders/matnwb?tab=readme-ov-file#tutorials"
             - text: "NWB YouTube Channel"
@@ -87,4 +89,4 @@ choosing_tools:
         - item: "If you need more flexibility or are comfortable with Python, consider NeuroConv"
         - item: "For custom data formats, complex conversions, or when developing NWB extensions, use PyNWB or MatNWB."
     content: "Remember to consult the provided documentation and tutorials for detailed guidance on using each tool"
----
+---