Add a text field for a discouragement reason

[ddbeck]

Talking to the MDN folks, they'd like a brief summary of why a given feature was marked discouraged.

They'd like this so that they can generate, from web-features, deprecation banners that correspond the most common cases on MDN, where there's mostly boilerplate text with a sentence explaining the reason for the deprecation.

This seems in-scope for what we're doing, like descriptions.

For example, for mutation-events:

Mutation events have poor performance and browser vendors plan to remove support for them in 2025.

Or for import-assertions we might have something like:

Import assertions are superseded by import attributes.

[captainbrosset]

I agree we should do this. We've kind of tried to stay away from adding too much information and content to web-features, because things on the web platform move a lot. But the kind of features we're talking about doing this for don't move anymore at all. So it feels pretty good for the project to act as a little summary of whatever is written at according_to, so that consumers can display:

This feature is discouraged: ${feature.discouraged.reason} (<a href="${feature.discouraged.according_to}">learn more</a>)