copilot: add GitHub Copilot instructions

copilot: add GitHub Copilot instructions

Github allows to add a instructions file to each repo
(.github/copilot-instructions.md) to improve the behavior
of Copilot Reviews and Agent.

These instructions can also be customized per path, filetype, etc.:
https://docs.github.com/en/copilot/how-tos/configure-custom-instructions/add-repository-instructions

This commit was authored through a Github Agent session: https://github.com/ceph/ceph/tasks/edeca07b-eabd-477c-917a-a18e72a0e2c2

Co-authored-by: GitHub Copilot noreply@github.com
Generated-by: Claude Sonnet 4.5
Signed-off-by: Ernesto Puerta <epuertat@redhat.com>