Mention documentation answers alongside other Docker Skills examples (#26379)

## Description

Mention answering questions using official Docker documentation
alongside the existing Dockerfile and Compose examples in the Docker
Skills introduction. Keep the repository catalog as the place to find
individual skills and their descriptions.

## Related issues or tickets

Complements https://github.com/docker/docs/pull/26378, which advertises
Docker Skills in generated `llms.txt`.

## Reviews

Scoped rumdl and Vale checks pass with no errors, warnings, or
suggestions. `git diff --check` passes.

- [ ] Technical review
- [ ] Editorial review

---------

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

content/manuals/ai/skills/_index.md

@@ -9,10 +9,11 @@ params:
 ---
 
 Docker Skills are Docker's official, open-source guidance for compatible AI
-coding agents. They help agents with Docker tasks, such as improving a Dockerfile
-or setting up a Compose application. The collection evolves and doesn't cover
-every Docker product. Browse the [repository catalog](https://github.com/docker/skills#readme)
-for available skills and their descriptions.
+coding agents. They help agents with Docker tasks, such as improving a Dockerfile,
+setting up a Compose application, or answering questions using official Docker
+documentation. The collection evolves and doesn't cover every Docker product.
+Browse the [repository catalog](https://github.com/docker/skills#readme) for
+available skills and their descriptions.
 
 ## Get started