Update documentation for new endpoint naming

[aram356]

Description

Minor documentation updates to align with new endpoint naming conventions.

Changes

• SEQUENCE.md: Update flow diagram endpoint /first-party/ad → /ad/render
• crates/common/README.md: Change "first-party proxy" → "proxy" for clarity
• docs/guide/error-reference.md: Update example curl command to use /ad/render
• docs/guide/what-is-trusted-server.md: Change "3rd party JS" → "external JS"

Related

• PR: Update documentation for new endpoint naming #245
• Parent: Unable to render prebidjs ads with NextJS #179