Search: document dynamic attributes in index definitions

[fractal3000]

The xref:search:index-definitions.adoc page did not mention dynamic attributes, although the Search add-on supports indexing them since Jmix 2.8 (jmix-framework/jmix#829) and there is a dedicated Dynamic Attributes in Search page. This adds the missing references and also documents native field configuration in the programmatic-mapping example.