Clarification on Environment variables

### Existing documentation URL(s)

https://developers.cloudflare.com/pages/configuration/build-configuration/

### What changes are you suggesting?

The documentation refers to Settings > Environment variables

However, the UI calls it "Variables and Secrets" and places this section _outside_ the Build section.

### Additional information

Also, currently variables set using this UI (as a secret or plain text variable) are not available during build. I ended up having to resort to customizing the build command to include the environment variables there.

Would be nice if the documentation could state the expected behavior, since it's a little ambiguous at the moment.