[docs][charts] Revise the Bar and Gauge docs and begin standardizing Chart doc structure

[mapache-salvaje]

When users make their way over to the Charts from Material UI, there should be a consistent, predictable, and more or less seamless experience.

This PR revamps the content and the structure of the Bar and Gauge component pages to better align with the current standards for Material UI demo pages—for examples, see:

• https://mui.com/material-ui/react-snackbar/
• https://mui.com/material-ui/react-alert/
• https://mui.com/material-ui/react-divider/

The Overview / Demos pattern feels strange to me, and doesn't match up with the patterns for any other products. I propose merging the Demos into the Overview pages and renaming them "Common examples" as we've done in Joy UI docs.

We can use this PR to align on what may work best for the Charts as a whole, and then I'll apply the final pattern here to the rest of the pages in a follow-up.

Here's the template I'm starting from:

## Introduction

Brief explanation of what it is and how it works.

### Usage (optional)

If necessary, compare and contrast with other similar components that may (seemingly) have overlapping use cases.

## Basics

Component import for easy copy-pasting; optionally, further details on how to set it up.

### {{ Basic features }}

h3s under Basics describe how to use built-in props.

## Customization

### {{ Other common use cases }}

h3s under Customization describe how to implement other common use cases that go beyond built-in props.

## Composition

Subcomponent imports; details on how to compose your own custom version of the component.

## Common examples (optional)

### {{ Real-world-ish use case }}

h3s under Common examples describe use cases that are closer to the real world and are focused on demos.

## Accessibility

### {{ A11y guidance }}

h3s under Accessibility describe individual requirements and best practices.

[mui-bot]

Deploy preview: https://deploy-preview-16500--material-ui-x.netlify.app/

Updated pages:

• docs/data/charts/bars/bars.md
• docs/data/charts/gauge/gauge.md

Generated by 🚫 dangerJS against b2b937b

[codspeed-hq]

CodSpeed Performance Report

Merging #16500 will not alter performance

_{Comparing mapache-salvaje:charts-editing (b2b937b) with master (1ffe360)}

Summary

✅ 6 untouched benchmarks

[github-actions]

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