[Naomi's Sprint] Add Section Headers to 400-499 Word Lessons

Following up on https://github.com/freeCodeCamp/freeCodeCamp/issues/70127, the 133 lectures below need section headers to break up long blocks of prose and make them easier to scan.

Tick each lecture off as its section headers are added. Each checkbox links to the lecture, with a source link identifying the exact file.

Acceptance Criteria

• Add short, Title Case ## section headers before each major subtopic in the lecture body (# --description-- or # --interactive--, as used by the file).
• Follow the natural structure of the content without rewriting the existing prose.
• Use How Does the Sort Method Work? as the heading example.
• Preserve the front matter, code blocks, and # --questions-- section unchanged.
• Ensure each lecture still reads clearly from beginning to end.

responsive-web-design-v9

• What Is the CSS Box Model, and How Does It Work? (source)
• What Are Design Briefs and How Do Developers Work with Them? (source)
• What Is CSS, and What Is Its Role on the Web? (source)
• How Do Width and Height Work? (source) - @Akshat026 - https://github.com/freeCodeCamp/freeCodeCamp/pull/70504
• What Are Some Common Accessibility Auditing Tools to Use? (source)
• How Do Background Image Size, Repeat, Position, and Attachment Work? (source) - @Akshat026 - https://github.com/freeCodeCamp/freeCodeCamp/pull/70505
• What Are the Different Types of Buttons, and When Should You Use Them? (source)
• What Are Best Practices for Working with Images in Your Designs? (source)
• What Is the aria-describedby Attribute, and How Does It Work? (source)
• How Do You Display Abbreviations in HTML? (source)
• What Is the RGB Color Model, and How Does the RGB Function Work in CSS? (source) - @Akshat026 - https://github.com/freeCodeCamp/freeCodeCamp/pull/70506
• What Is Client-Side Form Validation in HTML Forms, and What Are Some Examples? (source) - @tracemycodes
• How Do the Different list-style Properties Work? (source)
• What Are Best Practices for Hiding Content So It Doesn't Become Inaccessible? (source)
• What Are Inline, Internal, and External CSS, and When Should You Use Each One? (source)
• What Is the Attribute Selector, and How Can It Be Used to Target Links with the href and title Attributes? (source)
• What Are Absolute Units in CSS, and When Should You Use Them? (source)
• What Are Common Use Cases for Using Floats, and How Do They Work? (source)
• What Are IDs and Classes, and When Should You Use Them? (source)
• How Do You Style the Different Link States? (source)
• What Are CSS Animations, and How Do They Work? (source)
• How Does Alignment Work in Design? (source)
• What Is a Background Gradient, and How Does It Work? (source)
• What Are Some Common Tools Developers Should Know About That Are Used by Designers in the Industry? (source)
• What Are Some Best Practices for Working with Typography in Your Designs? (source)
• What Are Best Practices for Designing Cards? (source)
• How to Use the DOM Inspector and DevTools to Debug and Build Your Projects (source)
• What Are Description Lists, and When Should You Use Them? (source)
• Why Are Default Link Styles Important for Usability on the Web? (source)
• How Can You Use the DevTools Inspection Tool and CSS Validators to Debug Your CSS? (source)
• Why Is It Important to Have Good Structural Hierarchy? (source)
• What Are Best Practices for Designing Infinite Scrolls? (source)
• What Are Some Tools to Check for Good Color Contrast on Sites? (source)
• How Can You Effectively Work With Your Keyboard, Mouse, and Other Pointing Devices? (source)
• What Are Good Ways to Make Audio and Video Content Accessible? (source)
• What Are Screen Magnifiers Used For? (source)
• How Do Forms, Labels, and Inputs Work in HTML? (source)
• What Is Margin Collapsing, and How Does It Work? (source)
• What Are SVGs, and When Should You Use Them? (source)
• What Are CSS Custom Properties, and How Do They Work? (source)
• What Is the Difference Between Inline and Block-Level Elements in CSS? (source)
• What Is Progressive Disclosure? (source)
• What Are vh and vw Units, and When Should You Use Them? (source)
• What Are Alternative Pointing Devices Such as Trackballs, Joysticks, and Touchpads Used For? (source)
• What Are the Different Types of Tools Professional Developers Use? (source)

javascript-v9

• What Are Comments in JavaScript, and When Should You Use Them? (source) - @aliyasyeddd
• How Does the addEventListener Method Work? (source)
• What Are Linters and Formatters, and How Can They Help You with Code Consistency? (source)
• What Is the For...in Loop, and When Should You Use It? ([source](https://github.com/freeCodeCamp/freeCodeCamp/blob/main/curriculum/challenges/engli