Fix the margin overflow in the architecture diagram

Mermaid's default config option about flowcharts makes the big
ones have a top/bottom margin, see upstream issue
https://github.com/mermaid-js/mermaid/issues/1758

Since mermaid 8.6.0, we can apply an init directive and fix this
https://mermaid-js.github.io/mermaid/#/8.6.0_docs?id=init

doc/development/architecture.md

@@ -111,6 +111,7 @@ https://docs.google.com/drawings/d/1fBzAyklyveF-i-2q-OHUIqDkYfjjxC4mq5shwKSZHLs/
 ### Component diagram
 
 ```mermaid
+%%{init: {"flowchart": { "useMaxWidth": false } }}%%
 graph LR
   %% Anchor items in the appropriate subgraph.
   %% Link them where the destination* is.