feat: version-aware homepage install card

When the selected Boost version is not the latest, the homepage's
tabbed install card collapses to a short paragraph and a teal
"See Documentation" CTA inside the same card chrome. Tabs, panels,
dropdowns, and the per-option <style> block are skipped entirely.

The non-latest signal is selected_version_is_non_latest from the
existing selected_version context processor (URL slug -> cookie ->
most-recent fallback), so the card flips automatically when the user
picks a non-latest release from the navbar dropdown. No view changes
needed.

The older variant tightens the outer card gap to space-large so the
simpler content doesn't leave an awkward vertical gap, and neutralizes
the site-wide p { @apply py-5 } on the description.

Author: julioest

static/css/v3/install-card.css

@@ -170,6 +170,29 @@
 
 /* Per-option visibility rules are generated by Django in _install_card.html */
 
+/* ── Older-version variant ───────────────────────────────── */
+.install-card--older {
+  gap: var(--space-large);
+}
+
+.install-card__older-message {
+  margin: 0;
+  /* Override the site-wide `p { @apply py-5 }` from frontend/styles.css */
+  padding-top: 0;
+  padding-bottom: 0;
+  color: var(--color-text-secondary);
+  font-size: var(--font-size-base);
+  line-height: var(--line-height-loose);
+}
+
+.install-card__older-cta {
+  display: flex;
+}
+
+.install-card__older-cta .btn {
+  flex: 1 0 0;
+}
+
 /* ── Responsive ───────────────────────────────────────────── */
 @media (max-width: 767px) {
   .install-card__dropdown {

templates/v3/includes/_install_card.html

@@ -37,6 +37,24 @@
 {% endcomment %}
 {% with resolved_title=title|slugify|default:"install" %}
 {% with card_id=card_id|default:resolved_title %}
+{% if selected_version_is_non_latest %}
+<section class="install-card install-card--older card"
+         aria-labelledby="{{ card_id }}-heading">
+  <div class="card__header">
+    <h2 id="{{ card_id }}-heading" class="card__title install-card__title">
+      {{ title|default:"Install Boost and get started in your terminal." }}
+    </h2>
+  </div>
+  <div class="install-card__body px-large">
+    <p class="install-card__older-message">
+      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 %}
+    </div>
+  </div>
+</section>
+{% else %}
 {# Per-option visibility rules (generated from data, complements static rules in install-card.css) #}
 <style>
   {% for opt in install_card_pkg_managers %}
@@ -131,5 +149,6 @@ <h2 id="{{ card_id }}-heading" class="card__title install-card__title">
     </div>
   </div>
 </section>
+{% endif %}
 {% endwith %}
 {% endwith %}