All exercises: Changed the structure of the exercises to better align with course curriculum

[oceanthunder]

Because

Gives a better structure to the exercises by dividing the exercises as they occur in TOP.

├── advanced-html-css
│   └── animation
│       ├── 01-button-hover
│       ├── 02-pop-up
│       └── 03-dropdown-menu
├── foundations
│   ├── block-and-inline
│   │   ├── 01-margin-and-padding-1
│   │   └── 02-margin-and-padding-2
│   ├── cascade
│   │   └── 01-cascade-fix
│   ├── flex
│   │   ├── 01-flex-center
│   │   ├── 02-flex-header
│   │   ├── 03-flex-header-2
│   │   ├── 04-flex-information
│   │   ├── 05-flex-modal
│   │   ├── 06-flex-layout
│   │   └── 07-flex-layout-2
│   └── intro-to-css
│       ├── 01-css-methods
│       ├── 02-class-id-selectors
│       ├── 03-grouping-selectors
│       ├── 04-chaining-selectors
│       └── 05-descendant-combinator
└── intermediate-html-css
    ├── advanced-grid
    │   ├── 01-responsive-holy-grail
    │   └── 02-holy-grail-mockup
    └── positioning-grid
        └── 01-basic-holy-grail

This PR

• It divides the exercises in 3 directories

Issue

Closes #593

Additional Information

No

Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project Contributing Guide
• [x] The title of this PR follows the location of change: brief description of change format, e.g. 01-flex-center: Update self check
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [x] If this PR addresses an open issue, it is linked in the Issue section
• [x] If applicable, I have ensured that the TOP solution files match the Desired Outcome image

[oceanthunder]

Thanks for doing this.

No, thank you!