Add breaking change page for the soft hyphen character - #14048

Description of what this PR is changing or adding, and why:

Adds a breaking change page for flutter/flutter#196152. It adds a hyphenCharacter parameter that chooses the string drawn at a soft hyphen line break, and TextStyle.getParagraphStyle gained the parameter, which breaks classes that override it. The page covers the parameter and the migration for overrides. Rendering doesn't change unless the parameter is set.

Issues fixed by this PR (if any):

None.

PRs or commits this PR depends on (if any):

flutter/flutter#196152, which hasn't landed yet. This is a draft until it does, and I'll fill in the Timeline then.

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.