[generate-docc-reference] [GSoC 2025] Separate DocC Pages for Each Command

### Description
We’ve recently integrated the `generate-docc-reference` plugin into the `SwiftArgumentParser` project. One of the key improvements outlined in our project goals is to refine this plugin, enabling it to _generate separate documentation pages_ for each command and organize them into a _well-structured set of files_. For now the plugin only generates an **article** for **the main command**. As articles are considered good for communicating the general purpose and overview of the project, we should create **seperate pages** for each command to get low level on explanations and how things work.

### Proposed Structure
We can follow the structure below in order to expand our documentation generation further.

### High Level Overview of the Structure
- **Separation of Command Documentation**
The documentation will be split into individual pages for each command, with each page covering the details of the command, its flags, options, and arguments. This will provide clear, in-depth explanations for every command, rather than a single generic overview.

- **Hierarchical File Structure**
We will adopt a folder-based structure where each command, flag, option, and argument has its own dedicated markdown file. If a command has subcommands, they will also be structured into their own folders and documented similarly, ensuring clarity for users with more complex command-line tools.

- **Metadata Categorization**
To maintain organization, metadata will be added to each markdown file, categorizing them as command, flag, option, or argument. This will help categorize the components and provide meaningful context for users when navigating the documentation.

- **Metadata Files**
Relevant JSON files, such as topics.json, will be included in the root directory to define the overall structure of the documentation catalog.

### Next Steps
1. Since the contribution guidelines encourage small improvements and changes, we can start by creating directories and separate files for each command.
2. Then we can create documents for argument, options and flags, listing all related properties to the command.
3. Next, we can proceed to add the root metadata files like topics.json for the documentation catalog.
4. Finally, we can focus on structuring the metadata for the arguments, options, and flags for each command.
5. Then we can get low level and design the structure of our documentation files by solving the mentioned issues below:
- https://github.com/apple/swift-argument-parser/issues/721
- https://github.com/apple/swift-argument-parser/issues/722