Improve starters v25

[peholmst]

This PR removes the tabs from the Starters page, showing the different alternatives of starting a Vaadin project in separate subpages in the navigation tree.

It also moves the Vaadin Start documentation from the Tools section into the Starters section and renames it to Playground, as that is the term currently used at Vaadin Start.

[github-actions]

AI Language Review

• In the file "gradle-usage.adoc", a section reference link is broken: <<{articles}/getting-started/starters/gradle#, Starting a Vaadin Project with Gradle>>. It currently shows as {articles}/getting-started/starters/gradle# instead of rendering correctly.
• Ensure consistent use of spaces and punctuation in section headings and notes. For instance, there are some inconsistencies in applying punctuation or spacing around conjunctions and surrounding brackets.
• Check for consistent use of filename syntax (e.g., [filename]build.gradle) throughout the document to enhance clarity and uniformity.