Integrate an alpha release of the theme in the project

[jolelievre]

This is the step to integrate an alpha version in the core, the theme will be accessible but not used as the default one

• Release the theme with the latest changes into a 0.2.0 version (see this PR Release 0.2.0 hummingbird#600 - the version needs to be bumped to 0.2.0)
• Prepare a page in devdocs that explains the technical constraints for this theme (can be empty at first but we need the URL for the following task) The page can later be enriched
  • Explain that the theme is based on bootstrap5 and won't be compatible with all modules until they handle the theme
  • Explain the trick about integrating the Bootstrap Compatibility Layer (we still need to decide if it should be in the core, in the theme or in the module)
• Integrate hummingbird 0.2.0 into composer, adapt UI tests that don't need to install the theme anymore, integrate a warning popup when the theme is enabled to warn about bootstrap with a link to devdoc for more details (also see this issue Flag indicating the use of a new version of Bootstrap for developers hummingbird#581)

[Hlavtox]

A big issue is this that classic is hardcoded in several places in the code, including the translation system - #35299