chore: add documentation for Vaadin Executor

[mcollovati]

Fixes #4344

[github-actions]

AI Language Review

• In the "title" section, the current title "Configuration" is used twice; it might be redundant. Consider differentiating the titles to provide clarity, such as specifying the document context (e.g., "Spring Configuration Details").
• In the "Vaadin Executor" section, it would be beneficial to provide a brief introduction or context to its usage before diving into accessing and using the executor. This could help readers understand its importance and context in Vaadin applications.
• Ensure consistency in spacing within the code snippets, especially regarding inline comments and code alignment for better readability.
• Consider adding cross-references for sections that are interlinked within the document to directly link readers to the relevant sections, enhancing navigability.
• Verify all links and references for accuracy and ensure that they point to valid resources or sections within the documentation.