combine all CLI command references into one page

# Background

Chisel is a developer tool for extracting highly customized and specialized package slices of Ubuntu packages to create compact, secure software.

The Chisel documentation currently spreads CLI command reference content across multiple pages. Consolidating this material into a single page would make it easier to browse, link to, and maintain.

This is a duplicate from canonical/open-documentation-academy#379.

# Task

Combine all CLI command references into one page in the Chisel documentation.

Suggested steps:
* Create a single command reference page that contains the complete CLI reference.
* Ensure headings, anchors, and page structure make the combined content easy to navigate.
* Update internal links that currently point to split reference pages.

Once you’re assigned this issue, please open a pull request directly into [canonical/chisel-docs](https://github.com/canonical/chisel-docs). In order to contribute, you must sign the [Canonical contributor license agreement](https://ubuntu.com/legal/contributors).

# What you will learn

* Organizing CLI documentation for usability and maintenance.
* Updating cross-links in a docs site.
* Working with GitHub workflows.

# Outcome

The CLI reference material is available from one well-structured page, and outdated internal links are updated accordingly.

# Resources

* [Chisel repository](https://github.com/canonical/chisel-docs)
* [Reference index page](https://github.com/canonical/chisel-docs/blob/main/docs/reference/index.md)
* [CLI command index page](https://github.com/canonical/chisel-docs/blob/main/docs/reference/cmd/index.md)

# Mentor

The mentor for this issue is @asanvaq. They are the Technical Author for Rockcraft and Chisel.

Please feel free to reach out to the mentor on [Matrix](https://matrix.to/#/@sanchovaquera:ubuntu.com) if you have further questions.

Start by reading docs/reference/index.md and docs/reference/cmd/index.md to map the existing CLI reference pages and their links. Consolidate the command reference content into one navigable page, update links that target the split pages, and verify the reference index and internal links point to the combined page.

Written by the indexing model from the issue text.