docs: Restore blockdiag diagrams

- Replace simple blockdiag diagrams with Mermaid.
- Use the rendered SVG for other blockdiag diagrams for now.

Change-Id: I992a0e8314520721540d3e4411cf6810c2f3c79e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/60060
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Rob Mohr <mohrr@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>
diff --git a/pw_log/BUILD.gn b/pw_log/BUILD.gn
index d639762..14333a9 100644
--- a/pw_log/BUILD.gn
+++ b/pw_log/BUILD.gn
@@ -114,5 +114,8 @@
     "docs.rst",
     "protobuf.rst",
   ]
-  inputs = [ "log.proto" ]
+  inputs = [
+    "example_layer_diagram.svg",
+    "log.proto",
+  ]
 }