doc: remove "end user"

We use "end user" in ambiguous ways. Sometimes we mean the developer, and sometimes we mean the application user. Use "developer" where developer is meant.

Refs: https://docs.microsoft.com/en-us/style-guide/a-z-word-list-term-collections/u/user-end-user

Checklist

• documentation is changed or added
• commit message follows commit guidelines