Fix links in PMM contribution guide

* What should be done:

Currently, our contribution guideline contains a lot of links to archived repositories like pmm-admin, pmm-agent, etc. So we need to go through them and point to `pmm` repositories 

* Benefit:

Up-to-date information in our contribution guideline.

• • Suggested implementation:*

Go through contribution guidelines and check links

• • Details:*

Entrypoint to start working on https://github.com/percona/pmm/blob/main/CONTRIBUTING.md