Add Concerto v2 migration docs

[ ] Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
[ ] Vital features and changes captured in unit and/or integration tests
[ ] Commits messages follow AP format
[ ] Extend the documentation, if necessary
[ ] Merging to master from fork:branchname
[ ] Manual accessibility test performed
- [ ] Keyboard-only access, including forms
- [ ] Contrast at least WCAG Level A
- [ ] Appropriate labels, alt text, and instructions

accordproject / techdocs