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.
I had to change the title format here, the rst compiler was choking on the following error when adding the additional sublevel for the byte-sized section:

```
$ rst2html CONTRIBUTING.rst
CONTRIBUTING.rst:146: (SEVERE/4) Title level inconsistent:

===========================
Expectations We Have of You
===========================
Exiting due to level-4 (SEVERE) system message.
```