refactor: store documentation content in text files

[uAtomicBoolean]

Currently, the documentation takes around 45-50 minutes to compile on my computer due to all the documentation pages.
The documentation content could be stored in YAML or JSON files and loaded during startup, and then the application would get the content for the current page with a callback.
The drawback from doing this is that there would be a huge if condition to manage all the widgets' presentations.

To do after Slint's 1.16 release:

• Use the slint structs to load the yaml files' contents (Implement serde Serialize/Deserialize for ModelRc slint-ui/slint#11054).
• Refactor the DocSection description to use the new StyledText official widget.

[uAtomicBoolean]

Postpone this issue to wait for Slint 1.16 as it will come with the new StyledText widget with both inline code and link builtins.
This will allow me to refactor the DocSection widget and will greatly ease the use of text files for the documentation text files.