Add numbering to some diagrams, update README to include guidance on diagrams (#436)

Author: devalog

README.md

@@ -53,6 +53,7 @@ Keep the following principles in mind:
 - **Write in clear, concise language**, using [active voice](https://developers.google.com/style/voice) whenever possible. Keep sentences and paragraphs short and to the point. Be conversational and friendly tone. Stay away from jargon as much as you can.
 - **Use [Fern’s documentation components](https://buildwithfern.com/learn/docs/writing-content/components/overview)** whenever you can.
 - **When editing an existing page** - Match the existing heading structure, tone, and level of detail to ensure your changes integrate as seamlessly as possible.
+- **Use diagrams when it makes sense** – Show, don't tell! Use [Mermaid](https://buildwithfern.com/learn/docs/writing-content/markdown#diagrams), a Markdown-like diagramming syntax, to illustrate a workflow. 
 
 > "Break any of these rules sooner than say anything outright barbarous."
 > 

fern/products/docs/pages/authentication/sso.mdx

@@ -22,6 +22,7 @@ Fern's SSO integration allows your team to use their existing corporate credenti
 
 ```mermaid
 sequenceDiagram
+    autonumber
     participant U as User
     participant F as Fern Docs
     participant I as Identity Provider

fern/products/docs/pages/enterprise/self-hosted.mdx

@@ -41,6 +41,7 @@ Fern provides your documentation site as a ready-to-run Docker container that yo
 
 ```mermaid
 sequenceDiagram
+    autonumber
     participant F as Fern
     participant C as Customer
     participant S as Customer Server