docs: complete the 2.0 migration guide (#431)

* docs: complete the 2.0 migration guide and release notes (#404)

* docs: tighten migration guide and link to docs site

Author: lesnik512

README.md

@@ -76,6 +76,7 @@ The following intentionally differ:
 ## Links
 
 - [Documentation](https://lite-bootstrap.modern-python.org)
+- [Upgrading to 2.x](https://lite-bootstrap.modern-python.org/migration/to-2.x/)
 - [PyPI](https://pypi.org/project/lite-bootstrap)
 - [License](https://github.com/modern-python/lite-bootstrap/blob/main/LICENSE)