The `ItemProcessor` section in reference docs should be a top level section

[fmbenhassine]

The ItemProcessor section is currently under Item Readers and Item Writers section and is not visible.

This section should be a top level one, next to item readers and writers in the documentation index.

The Validating Input section which describes a typical use case for item processors should also be part of this new ItemProcessor section.

[fmbenhassine]

Resolved with #3698 .