Update index.adoc

[peholmst]

Explain new branches

[github-actions]

AI Language Review

The main branch description has been updated for clarity regarding versioning, specifically identifying main as related to the next unreleased version of Vaadin, and that released versions use their respective branches, such as v24. This change improves the accuracy of the documentation but introduces specificity that needs maintaining if the process changes. Ensure that future updates reflect any alterations in branch management practices.