[Content]: Improvements for the session docs

[amirhhashemi]

📚 Subject area/topic

Session

📋 Page(s) affected (or suggested, for new content)

https://docs.solidjs.com/solid-start/advanced/session

📋 Description of content that is out-of-date or incorrect

I have a few suggestions for improving the session docs.

• The docs would benefit from a clear definition of "session" and examples of common use cases (e.g., authentication, user preferences).
• The introduction ("When user information is required...") is confusing. Rephrasing to clarify why we need sessions would be helpful.
• cache is deprecated and should be updated.
• The docs states Nitro provides session helpers, but they are re-exported from h3 via Vinxi.
• Examples:
  • File paths starting with "/" are confusing. Clarifying their context (project root or src) would be good.
  • FormData seems unnecessary in the last example.
  • Examples can be more readable by simply adding empty lines. The code is very compact.

I'd be happy to send a PR with proposed changes. I would probably re-write it completely. Would you be open to that?

🖥️ Reproduction in StackBlitz (if reporting incorrect content or code samples)

No response