add missing values handling templates for model documentation

[focardozom]

Add roxygen2 templates to document how models handle missing values #812

This commit create three files under man-roxygen/ that describe
how different model engines handle missing values:

• na_casewise_delete.R: for models that silently drop rows with NAs
• na_handled.R: for models that natively support NAs (e.g., rpart)
• naerrors.R: for models that fail when NAs are present

These templates can be added to individual model documentation.

[topepo]

Thanks! This is great.