CONTRIBUTING: Guidelines for numbering and naming notebooks

Synthesizing what we already have + the discussion from #236:

* Reference documents (those we do not intend to cover during training) should be prefixed with `00-`. If there are multiple references for a document, `00` should be followed by a lowercase letter that reflects their order, e.g., `00a-`, `00b-`.
* Notebooks that are covered with participants are sequentially ordered, e.g., `01-`, `02-`.
* These become `-live.Rmd` when we run `make-live.R`.
* Exercise notebooks should be prefixed with `exercise_-` and numbered sequentially. If there are multiple notebooks for a given topic, use lowercase letters after the number. For example, from the

Start with the existing CONTRIBUTING guidance and review the discussion in #236 for the agreed naming rules. Check how make-live.R handles the -live.R conversion, then document the 00-, numbered, and exercise_ naming conventions, including the ordering examples, in the relevant contribution guidance.

Written by the indexing model from the issue text.