[jfx20] RFR: 8290863: Update the documentation of Virtualized controls to include the best practice of not using Nodes directly in the item list

[Ajit Ghaisas]

This PR adds a warning about inserting Nodes directly into the virtualized containers such as ListView, TreeView, TableView and TreeTableView. It also adds code snippets showing the recommended pattern of using a custom cell factory for each of the virtualized control.

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Commit messages:
 - Doc updates

Changes: https://git.openjdk.org/jfx/pull/995/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=995&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290863
  Stats: 308 lines in 5 files changed: 302 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/995.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/995/head:pull/995

PR: https://git.openjdk.org/jfx/pull/995