doc: add 3.4.3 release note section, document new SpiderMonkey support

[janl]

Overview

Testing recommendations

Related Issues or Pull Requests

Checklist

• [ ] Code is written and works correctly
• [ ] Changes are covered by tests
• [ ] Any new configurable parameters are documented in rel/overlay/etc/default.ini
• [x] Documentation changes were made in the src/docs folder
• [ ] Documentation changes were backported (separated PR) to affected branches

[janl]

Holding in draft until SM>91 support was added to 3.4.x

[big-r81]

@janl Do we want to close this, because it was superseeded by 3664cb0f37f16d49891d83526e0f63cbe5344ac2?