Document a capability table for the native Delta reader

### What is the problem the feature request solves?

Follow-up from the native Delta Lake scan review (#5365). The experimental native Delta reader needs a capability table describing native support and fallback boundaries for column mapping by ID, mapped nested schemas, CDF, and generated row metadata, and clarifying that constant file metadata and consumed row indexes have different support rules. Follow-up issues should be linked so users can distinguish deliberate fallback from planned extensions.

### Describe the potential solution

Extend docs/source/user-guide/latest/delta.md with the table once #5365 lands.

### Additional context

Raised in the #5365 review discussion.

Read the native Delta scan review in #5365, then inspect docs/source/user-guide/latest/delta.md once that review lands. Add a capability table covering the specified column-mapping, nested-schema, CDF, generated-metadata, constant-file-metadata, and consumed-row-index rules, and link follow-up issues so deliberate fallbacks are distinct from planned extensions.

Written by the indexing model from the issue text.