Documentation: Clarify approach for header setting/getting documentation

[coderabbitai]

Question raised by @SandPod:

In the new page we don't really talk about setting and getting headers, do we defer this to the Relic docs?

Context:
The typed-headers.md page was removed and replaced with a general page for accessing request data (added as 03). We need to decide whether header documentation should be:

• Included in the Serverpod docs
• Deferred to the Relic documentation

References:

• PR: docs: Add documentation for Serverpod 3 WebServer" #342
• Comment: docs: Add documentation for Serverpod 3 WebServer" #342 (comment)
• Requested by: @nielsenko