Add Swift.org and Stanford tutorials to Swift section

[Licoler]

Description

Added new high-quality, project-based tutorials for Swift and SwiftUI to expand the currently limited Swift section.

Types of changes

• Content Update (change which fixes an issue or updates an already existing submission)

Checklist:

• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• I have made checks to ensure URLs and other resources are valid

[tuvtran]

Thanks for these two solid additions! Both are great free project-based resources, so I've applied them to the Swift section in commit 23cd083 (the diff had a stray whitespace change on an unrelated line, so I applied it manually). Appreciate the contribution!