New discouraged MathML 3.0 feature

[captainbrosset]

This PR fixes #2513 by creating a new discouraged feature which contains the remaining deprecated MathML BCD keys.

[captainbrosset]

@eerii I'd love your help with this. Would you, by any chance, be willing to review the names and descriptions of the 2 yaml files in this PR.
This is for updating/creating 2 features on the web-features repository to cover MathML. One of these features already exists, and is meant to cover MathML Core. While the other feature is being added by this PR, and covers MathML full, and it comes with a message to discourage use in web content.
Do these descriptions/scopes make sense to you?

[eerii]

Looks good! The descriptions are very explanatory. I read the attached issues and I agree that "discouraged" (with the reason you put in the yaml) is more accurate than "deprecated". And grouping them by belonging to MathML Core or not makes sense too. Thank you for making this :)

[captainbrosset]

@ddbeck could you please review? Also see @eerii's (yes, this Eri 😀) comment above.