Update prompt and instructions files (#8318)

* Update prompt files

* Update custom instructions

* Update instructions examples

* Incorporate feedback

* Update example

* Update path

* Update path

* Update docs/copilot/copilot-customization.md

Co-authored-by: Mine Starks <[email protected]>

* Clarify examples of custom instructions for code generation in documentation

* Reorder sections

---------

Co-authored-by: Mine Starks <[email protected]>

Author: ntrogh