Add OSS and RubyGems sponsor sections to Open Source page (#37)

Adds sponsor logo sections for:
- Open Source Sustaining Membership Sponsors
- RubyGems Supporters Plus

Uses existing sponsor grid styles and Ghost page tags.

<img width="720" height="487" alt="os_rubygems_sponsor_update"
src="https://github.com/user-attachments/assets/c4373099-761e-415c-b12f-18becadf903e"
/>

---------

Co-authored-by: Daniel Brinkley <dbrinkley@Daniels-MacBook-Air.local>
Co-authored-by: Jim Remsik <jremsikjr@gmail.com>

Author: 3 people

custom-modern-open-source.hbs

@@ -91,7 +91,35 @@
       {{/foreach}}
       {{/get}}
       </div>
-  </div>
+      <div class="container">
+  <section class="sponsors">
+    <p class="sponsors__title">Our Open Source Sponsors</p>
+    <div class="sponsors__inner">
+      {{#get "pages" order="featured desc, title asc" filter="tags:hash-oss-sponsors" limit="all"}}
+        {{#foreach pages}}
+          {{#if feature_image}}
+            <img src="{{feature_image}}" alt="{{title}}" />
+          {{/if}}
+        {{/foreach}}
+      {{/get}}
+    </div>
+  </section>
+</div>
+
+<div class="container">
+  <section class="sponsors">
+    <p class="sponsors__title">RubyGems Supporters Plus</p>
+    <div class="sponsors__inner">
+      {{#get "pages" order="featured desc, title asc" filter="tags:hash-rubygems-sponsors" limit="all"}}
+        {{#foreach pages}}
+          {{#if feature_image}}
+            <img src="{{feature_image}}" alt="{{title}}" />
+          {{/if}}
+        {{/foreach}}
+      {{/get}}
+    </div>
+  </section>
+</div>
 {{/post}}
 {{#contentFor 'scripts'}}
 <script src="{{asset 'dist/post.js'}}" defer></script>