docs: clarify Environment API externalization options

[lilianakatrina684-a11y]

Fixes #20098

Summary:

• clarify in the Environment API guide that dependency externalization for server environments is configured under resolve, not build
• document that environments..resolve.external and environments..resolve.noExternal are the Environment API equivalents of the top-level ssr.external and ssr.noExternal options
• note that the top-level ssr.* options remain the backward-compatible alias for the default ssr environment

Verification:

• git diff --check
• npx prettier@3.8.1 --check docs/guide/api-environment.md docs/config/ssr-options.md