docs: use diagram images for image lifecycle

Documentation/devel/architecture.md

@@ -170,14 +170,7 @@ rkt commands like prepare and run, as a first step, need to retrieve all the ima
 
 This is done with the following chain:
 
-```
- -----------            -----------            ------------
-|           |          |           |          |            |
-|   Fetch   |--------->|   Store   |--------->|   Render   |
-|           |          |           |          |            |
- -----------            -----------            ------------
-
-```
+![image-chain](image-chain.png)
 
 * Fetch: in the fetch phase rkt retrieves the requested images. The fetching implementation depends on the provided image argument such as an image string/hash/https URL/file (e.g. `example.com/app:v1.0`).
 * Store: in the store phase the fetched images are saved to the local store. The local store is a cache for fetched images and related data.
@@ -186,20 +179,7 @@ This is done with the following chain:
 
 These three logical blocks are implemented inside rkt in this way:
 
-```
- ------------            ---------------                -------------                    ------------------------
-|            |          |               |              |             |     overlayfs    |                        |
-|  Fetchers  |--------->|  Image Store  |<-------------|  TreeStore  |<-----------------|  Stage1-2 fs contents  |
-|            |          |               |<----         |             |           -------|                        |
- ------------            ---------------      \         -------------           /        ------------------------
-                                               \                               /
-                                                \       -----------------     /
-                                                 \     |                 |   /
-                                                  -----| Direct stage1-2 |---
-                                                       |   renderer      |
-                                                       |                 |
-                                                        -----------------
-```
+![image-logical-blocks](image-logical-blocks.png)
 
 Currently rkt implements the [appc][appc-spec] internally, converting to it from other container image formats for compatibility. In the future, additional formats like the [OCI image spec][oci-img-spec] may be added to rkt, keeping the same basic scheme for fetching, storing, and rendering application container images.
 

Documentation/devel/image-chain.dot

@@ -0,0 +1,16 @@
+digraph G {
+    rankdir="LR";
+
+    graph [fontname = "helvetica"];
+    node [fontname = "Arial", fillcolor="#FFE599", style="filled"];
+    edge [fontname = "monospace"];
+
+    {
+        fetch [shape="node", label=<Fetch>]
+        store [shape="node", label=<Store>]
+        render [shape="node", label=<Render>]
+    }
+
+    fetch -> store
+    store -> render
+}

Documentation/devel/image-logical-blocks.dot

@@ -0,0 +1,21 @@
+digraph G {
+    rankdir="LR";
+
+    graph [fontname = "helvetica"];
+    node [fontname = "Arial", fillcolor="#FFE599", style="filled"];
+    edge [fontname = "monospace"];
+
+    {
+        fetchers [shape="node", label=<Fetchers>, pos="0,0!"]
+        image_store [shape="node", label=<Image Store>, pos="2,0!"]
+        tree_store [shape="node", label=<Tree Store>, pos="4,0!"]
+        fs_contents [shape="node", label=<Stage1-2 fs contents>, pos="6,0!"]
+        renderer [shape="node", label=<Direct Stage1-2 renderer>, pos="4,-1!"]
+    }
+
+    fetchers -> image_store
+    tree_store -> image_store
+    renderer -> image_store
+    fs_contents -> tree_store
+    renderer -> fs_contents [dir=none]
+}