Use downloaded PNGs and add image sources (#24)

* Use downloaded PNGs and add image sources

Signed-off-by: lucperkins <lucperkins@gmail.com>

* Remove extra image

Signed-off-by: lucperkins <lucperkins@gmail.com>

content/docs/_index.md

@@ -26,6 +26,11 @@ TiKV was originally inspired by [Google Spanner](https://ai.google/research/pubs
 
 The basic architecture of TiKV is described in the diagram below:
 
-{{< figure src="https://www.lucidchart.com/publicSegments/view/d6ff8e03-ed36-46b3-83b3-a1ce79db03a4/image.png" caption="The architecture of TiKV" alt="TiKV architecture diagram" width="70" number="1" >}}
+{{< figure
+    src="/img/basic-architecture.png"
+    caption="The architecture of TiKV"
+    alt="TiKV architecture diagram"
+    width="70"
+    number="1" >}}
 
 You can read more in the [Concepts and architecture](architecture) documentation.

content/docs/architecture.md

@@ -28,7 +28,7 @@ Transactional | A higher-level key-value API that provides ACID semantics | Mult
 The overall architecture of TiKV is illustrated in **Figure 1** below:
 
 {{< figure
-    id="d6ff8e03-ed36-46b3-83b3-a1ce79db03a4"
+    src="/img/basic-architecture.png"
     caption="The architecture of TiKV"
     alt="TiKV architecture diagram"
     width="70"
@@ -39,7 +39,7 @@ The overall architecture of TiKV is illustrated in **Figure 1** below:
 The architecture of each TiKV instance is illustrated in **Figure 2** below:
 
 {{< figure
-    id="f371145f-126d-42e0-99fe-2c06080419a9"
+    src="/img/tikv-instance.png"
     caption="TiKV instance architecture"
     alt="TiKV instance architecture diagram"
     width="60"