docs: Update compatible sources shortcode to link to source.md (#2919)

## Description

Currently the shortcode linked to the _index.md which is an empty page.
This fixes the above issue

## PR Checklist

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- [x] Make sure you reviewed

[CONTRIBUTING.md](https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
- [x] Make sure to open an issue as a

[bug/issue](https://github.com/googleapis/genai-toolbox/issues/new/choose)
  before writing your code! That way we can discuss the change, evaluate
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- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)
- [x] Make sure to add `!` if this involve a breaking change

🛠️ Fixes #<issue_number_goes_here>

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: dishaprakash

.hugo/layouts/shortcodes/compatible-sources.html

@@ -1,5 +1,9 @@
-{{/* Automatically identify the "Native" source (Grandparent _index.md, since this is strictly used inside tools/) */}}
-{{ $nativeSource := .Page.Parent.Parent }}
+{{/* Automatically identify the "Native" source (Grandparent source.md, since this is strictly used inside tools/) */}}
+{{ $nativeSource := .Page.Parent.Parent.GetPage "source.md" }}
+
+{{ if not $nativeSource }}
+  {{ $nativeSource = .Page.Parent.Parent.GetPage "source" }}
+{{ end }}
 
 <div class="compatibility-section">
   <p>This tool can be used with the following database sources:</p>
@@ -19,11 +23,13 @@
       {{ end }}
 
       {{/* Process additional sources passed via the "others" parameter */}}
+
       {{ $others := .Get "others" }}
       {{ if $others }}
         {{ range split $others "," }}
           {{ $path := trim . " " }}
-          {{ $remotePage := site.GetPage $path }}
+          {{ $cleanPath := trim $path "/" }}
+          {{ $remotePage := site.GetPage (printf "%s/source.md" $cleanPath) }}
 
           {{ if $remotePage }}
           <tr>
@@ -38,4 +44,4 @@
       {{ end }}
     </tbody>
   </table>
-</div>
+</div>