Update redirects to use new default permanent behavior

- Remove all 'permanent: true' entries from docs.yml (now default)
- Update redirect documentation to reflect new default (permanent: true)
- Add example of using permanent: false for temporary redirects
- Update field descriptions per PR fern-api/fern-platform#4141

Co-Authored-By: [email protected] <[email protected]>

Author: devin-ai-integration[bot]