Some minor styling changes to Careers page

- Margin adjustments for desktop/mobile devices
- Reduce font size for `Powered by` text

Related to #50925

Signed-off-by: Cintia Sánchez García <[email protected]>

Author: cynthia-sg

content/en/careers/_index.html

@@ -10,10 +10,11 @@
     weight: 70
 ---
 
-<div align="center">
-  <iframe id="gitjobs" src="https://gitjobs.dev/embed?ts_query=kubernetes" style="width:100%;max-width:870px;height:100%;display:block;border:none;"></iframe>
-  <div>Powered by <a href="https://gitjobs.dev" target="_blank">GitJobs</a></div>
-  <p>&nbsp;</p>
+<div class="d-flex flex-column justify-content-center mt-4 mt-md-5 px-2 px-md-3 px-lg-0">
+  <iframe id="gitjobs" class="mx-auto" src="https://gitjobs.dev/embed?ts_query=kubernetes" style="width:100%;max-width:870px;height:100%;display:block;border:none;"></iframe>
+  <div class="mb-4 mb-md-5 mt-1 mx-auto">
+    <small>Powered by <a href="https://gitjobs.dev" target="_blank">GitJobs</a></small>
+  </div>
 </div>
 
 <script type="module">