feat: Add landing page archetype (#668)

This commit adds a new landing page archetype, which has the
ability to display various cards to highlight specific items. The
archetype includes inline guidance like other archetypes, including
explanations of new frontmatter parameters and a new card shortcode.

---------

Co-authored-by: Mike Jang <3287976+mjang@users.noreply.github.com>
Co-authored-by: Alan Dooley <a.dooley@f5.com>

Author: 3 people

archetypes/landing-page.md

@@ -0,0 +1,49 @@
+---
+# The title is the product name
+title: 
+# The URL is the base of the deployed path, becoming "docs.nginx.com/<url>/<other-pages>"
+url: 
+# The cascade directive applies its nested parameters down the page tree until overwritten
+cascade:
+  # The logo file is resolved from the theme, in the folder /static/images/icons/
+  logo:
+# The subtitle displays directly underneath the heading of a given page
+nd-subtitle: 
+# Indicates that this is a custom landing page
+nd-landing-page: true
+# Types have a 1:1 relationship with Hugo archetypes, so you shouldn't need to change this
+nd-content-type: landing-page
+# Intended for internal catalogue and search, case sensitive:
+# Agent, N4Azure, NIC, NIM, NGF, NAP-DOS, NAP-WAF, NGINX One, NGINX+, Solutions, Unit
+nd-product:
+---
+
+## About
+[//]: # "These are Markdown comments to guide you through document structure. Remove them as you go, as well as any unnecessary sections."
+[//]: # "Use underscores for _italics_, and double asterisks for **bold**."
+[//]: # "Backticks are for `monospace`, used sparingly and reserved mostly for executable names - they can cause formatting problems. Avoid them in tables: use italics instead."
+
+[//]: # "This initial section introduces the product to a reader: give a short 1-2 sentence summary of what the product does and its value to the reader."
+[//]: # "Name specific functionality it provides: avoid ambiguous descriptions such as 'enables efficiency', focus on what makes it unique."
+
+## Featured content
+[//]: # "You can add a maximum of three cards: any extra will not display."
+[//]: # "One card will take full width page: two will take half width each. Three will stack like an inverse pyramid."
+[//]: # "Some examples of content could be the latest release note, the most common install path, and a popular new feature."
+
+{{<card-layout>}}
+  {{<card-section showAsCards="true" isFeaturedSection="true">}}
+    {{<card title="<some-title>">}}
+      <!-- Each description should be roughly 10 words or less. -->
+    {{</card>}}
+    <!-- The titleURL and icon are both optional -->
+    <!-- Lucide icon names can be found at https://lucide.dev/icons/ -->
+    {{<card title="<some-title>" titleUrl="<some-url>" icon="<some-lucide-icon>">}}
+      <!-- Each description should be roughly 10 words or less. -->
+    {{</card>}}
+  {{</card-section>}}
+{{</card-layout>}}
+
+## Other content 
+
+[//]: # "You can add any extra content for the page here, such as additional cards, diagrams or text."