Make sure the Getting Started pages cover both Hilla and Flow

Once the PR https://github.com/vaadin/docs/pull/3279 has been merged, there will be a separate "Getting Started" section in the documentation. It currently contains the getting started pages for Flow, but as of 24.4, the steps should be the same for Hilla apps.  Therefore, these pages should all be reviewed to make sure they cover both Flow and Hilla.