docs: enrich landing pages (#2165)

Signed-off-by: Panos Vagenas <pva@zurich.ibm.com>

docs/concepts/docling_document.md

@@ -1,4 +1,4 @@
-With Docling v2, we introduce a unified document representation format called `DoclingDocument`. It is defined as a
+With Docling v2, we introduced a unified document representation format called `DoclingDocument`. It is defined as a
 pydantic datatype, which can express several features common to documents, such as:
 
 * Text, Tables, Pictures, and more

docs/concepts/index.md

@@ -1 +1,17 @@
-Use the navigation on the left to browse through some core Docling concepts.
+In this space, you can peek under the hood and learn some fundamental Docling concepts!
+
+Here some of our picks to get you started:
+
+- 🏛️ Docling [architecture](./architecture.md)
+- 📄 [Docling Document](./docling_document.md)
+- core operations like ✍️ [serialization](./serialization.md) and ✂️ [chunking](./chunking.md)
+
+👈 ... and there is much more: explore all the concepts using the navigation menu on the side
+
+<div class="grid" style="text-align: center">
+    <div class="card">
+        <img loading="lazy" alt="Docling architecture" src="../assets/docling_arch.png" width="75%" />
+        <hr />
+        Docling architecture outline
+    </div>
+</div>