[DOCS] Make landing page links relative (elastic#166314)

**Problem:**
The links on the Kibana docs landing page must be updated each major/minor release. We often forget this chore.

**Solution:**
Make the links relative so they always stay updated.

(cherry picked from commit 15d4e42)

docs/landing-page.asciidoc

@@ -71,8 +71,8 @@
       </a>
     </p>
     <p>
-      <a class="inline-block mr-3" href="https://www.elastic.co/guide/en/kibana/8.10/whats-new.html">What's new</a>
-      <a class="inline-block mr-3" href="https://www.elastic.co/guide/en/kibana/8.10/release-notes.html">Release notes</a>
+      <a class="inline-block mr-3" href="whats-new.html">What's new</a>
+      <a class="inline-block mr-3" href="release-notes.html">Release notes</a>
       <a class="inline-block mr-3" href="install.html">Install</a>
     </p>
   </div>