KTOR-6741 Part 1 - Replace "Creating HTTP API's tutorial" with a new tutorial "Handle requests and generate responses"

[vnikolova]

KTOR-6741

The "Creating HTTP APIs" tutorial will be replaced by two new tutorials:

1. Handle requests and generate responses. (orig. Understanding Routing and Requests)
2. Create RESTful APIs that generate JSON. (orig. Content Negotiation and REST)

In this PR: See preview on staging

• [x] Both new topics are added and placed in the navigation
• [x] "Creating HTTP APIs" is removed
• [x] Content is added for "Handle requests and generate responses" tutorial with screenshots for light and dark theme
• [x] An example project is added

The "Create RESTful APIs that generate JSON." topic is currently empty. It will be covered in a separate PR, to be merged to main later with this one.

[e5l]

some screenshots are flying :)