[Docs]: Rust: add instrumentation page scaffold

[svrnm]

What needs to be changed?

Create content/en/docs/languages/rust/instrumentation.md with short sections for Setup, Traces, Metrics, Logs, and Next steps.
Use Java docs structure as reference, but keep Rust-specific content only. Acceptance: page builds, appears in Rust nav, and links are valid.

Name + path of the page

content/en/docs/languages/rust/instrumentation.md

Additional context

No response