feat(opcp): create guide how to use api and get credentials

[ThibautOVH]

add OPCP
create guide how to use api and get credentials

[Jessica41]

Hello @ThibautOVH do you plan on adding more information to this guide? Thanks.

[ThibautOVH]

Hello @Jessica41,

No It's ok, we can push it in prod.
In addition we need to put this PR into production asap, to add new others docs for OPCP.

I just tried adding the index section. I hope I have understood correctly how to do this.

[Jessica41]

Thank you for your contribution @ThibautOVH