Migration guides for new default review environment

[olusoladavid]

This pull request adds migration guides for both the containerised-app and containerised-app-with-assets plugins, providing clear instructions for upgrading from v0.2.x to v0.3.x. The guides explain the introduction of a new default review environment and outline steps for updating configurations and deployments, highlighting breaking changes that may require action from developers.

Migration documentation updates:

• Added plugins/containerised-app/migration.md with a detailed migration guide for the containerised-app plugin, including steps to set up the new dedicated review environment and update plugin versions.
• Added plugins/containerised-app-with-assets/migration.md with a migration guide for the containerised-app-with-assets plugin, referencing the base plugin's migration process and emphasizing breaking changes.

Checklist:

• My branch has been rebased onto the latest commit on main (don't merge main into your branch)
• My commit messages are conventional commits, for example: feat(circleci): add support for nightly workflows, fix: set Heroku app name for staging apps too