Docs: Pin markdown and jinja2

docs/requirements.txt

@@ -1 +1,3 @@
+jinja2==3.0.3
+markdown==3.3.7
 mkdocs==1.0.4

docs/using/how-it-works.md

@@ -25,7 +25,7 @@ Proto and HTTP.
 ![Diagram with HTTP Bridge](communication_overview_http_app.svg)
 
 <!--
-```graphviz dot communication_overview_http_app.svg
+graphviz dot communication_overview_http_app.svg
     graph communication_overview_http_app {
       rankdir=LR;
       compound=true;
@@ -50,7 +50,6 @@ Proto and HTTP.
       websession -- bridge;
       bridge -- app;
     }
-```
 -->
 
 When an application can speak Cap'n Proto directly to Sandstorm, the HTTP
@@ -59,7 +58,7 @@ bridge is not needed.
 ![Diagram without HTTP Bridge](communication_overview_native_app.svg)
 
 <!--
-```graphviz dot communication_overview_native_app.svg
+graphviz dot communication_overview_native_app.svg
     graph communication_overview_native_app {
       rankdir=LR;
       compound=true;
@@ -82,5 +81,4 @@ bridge is not needed.
       proxy -- websession;
       websession -- app;
     }
-```
 -->