Declarative `RuleId` severity

[fwkoch]

We added RuleId severity to the docs here: https://github.com/jupyter-book/mystmd/pull/2333

However, that PR is scraping the code to infer severity. This script could be fragile if we modify RuleId usage in the code - it might be nicer if we defined severity in an explicit way. I made a more detailed comment here: https://github.com/jupyter-book/mystmd/pull/2333#pullrequestreview-3324307915