Update Baseline definition doc from owners group meeting

[ddbeck]

On Tuesday, 9 September 2025, the owners group had its periodic meeting. During the meeting, we decided to make the following changes to the Baseline definition doc:

• Advance the definition review date by 1 year.
• Add broad feature handling to the "Future considerations" section.
• Mention that we now have a facility for discouraged features, but it's not, strictly speaking, part of the definition of Baseline.

While we discussed some of the other future considerations (such as accessibility), we didn't think those conversations were ripe enough to make any changes to the definition immediately.