replace images with text in the mode-of-operation 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 includes a mode-of-operation page that currently relies on images to explain key parts of the workflow. This makes the content harder to search, quote, review, and maintain over time. Converting the visual explanations into text will improve accessibility and make the page easier to update.

There is already related context in [canonical/chisel-docs#31](https://github.com/canonical/chisel-docs/issues/31), which tracks past efforts in replacing images in `docs/explanation/mode-of-operation.md`.

This is a duplicate from canonical/open-documentation-academy#380

# Task

Replace the images in the mode-of-operation documentation page with equivalent text-based content (diagram), following the conclusions of the aforementioned issue and it's corresponding PR.

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

* Rewriting visual documentation into accessible text.
* Improving documentation accessibility and maintainability.
* Researching solutions based on past feedback.

# Outcome

The mode-of-operation page contains information in a diagram

# Resources

* [Chisel repository](https://github.com/canonical/chisel-docs)
* [Mode-of-operation page](https://github.com/canonical/chisel-docs/blob/main/docs/explanation/mode-of-operation.md)
* [Previous issue](https://github.com/canonical/chisel-docs/issues/31)
* [Previous pull request](https://github.com/canonical/chisel-docs/pull/40)

# 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.