docs: add KV values support to containers env-vars-and-secrets example

[Talador12]

Summary

Enhanced the containers env-vars-and-secrets example to include Workers KV support alongside existing secrets functionality. Added KV namespace setup, Wrangler configuration, and usage patterns for configuration data and feature flags.

Screenshots (optional)

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.