doc-byte-sized-tasks: Add link to the byte-sized tasks in CONTRIBUTING

Thanks to @adampalay, we now have a list of byte-sized tasks, which are
useful for new contributors who want a gentle introduction to the
platform. This PR adds a link to the tasks tagged as byte-sized in the
public tracker.
Merge pull request #4972 from open-craft/doc-byte-sized-tasks

doc-byte-sized-tasks: Add link to the byte-sized tasks in CONTRIBUTING