DocCards showing a category which has an associated doc should show that doc's description (used in generated-index)

### Have you read the Contributing Guidelines on issues?

- [X] I have read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#reporting-new-issues).

### Prerequisites

- [X] I'm using the latest version of Docusaurus.
- [X] I have tried the `npm run clear` or `yarn clear` command.
- [X] I have tried `rm -rf node_modules yarn.lock package-lock.json` and re-installing packages.
- [X] I have tried creating a repro with https://new.docusaurus.io.
- [X] I have read the console error message carefully (if applicable).

### Description

When showing a generated-index, or manually showing the DocCardList, a DocCard representing a Category where that category has an associated link or `_category_.*` should use the description associated with either of those items rather than just listing the number of items within it.

### Reproducible demo

https://docusaurus.io/docs/category/guides

### Steps to reproduce

1. See https://docusaurus.io/docs/category/guides
2. Note that the "Docs" category has four immediate children, but no description
3. View the Docs category
- **Note: This is a document, with a usable description which would be more useful in the DocCard**

### Expected behavior

A useful description is shown

### Actual behavior

The number of child documents is shown

### Your environment

N/A

### Self-service

- [ ] I'd be willing to fix this bug myself.