fix: link older-version install card to that version's docs

The "See Documentation" button on the older-version install card
was hardcoded to the current user-guide URL, so visitors looking
at an older version still got bounced to current docs. Point it at
selected_version.documentation_url so the link matches the version
the visitor is actually viewing.

Heads up: the original URL was a Getting Started page; the new URL
is the version-specific docs index, since older versions may not
have a per-version getting-started page.

Author: julioest

templates/v3/includes/_install_card.html

@@ -50,7 +50,7 @@ <h2 id="{{ card_id }}-heading" class="card__title install-card__title">
       You are viewing an older version. See the documentation for installation steps.
     </p>
     <div class="install-card__older-cta">
-      {% include "v3/includes/_button.html" with label="See Documentation" url="https://www.boost.org/doc/user-guide/getting-started.html" style="teal" aria_label="See documentation for installation steps" only %}
+      {% include "v3/includes/_button.html" with label="See Documentation" url=selected_version.documentation_url style="teal" aria_label="See documentation for installation steps" only %}
     </div>
   </div>
 </section>