Improve documentation and examples

Tracking issue for improving the egui docs. Other suggestions for improvements welcome!

All doc-examples need to be part of the doctests (i.e. they should never be marked `ignore`, though `no_run` can maybe sometimes be motivated).

# egui
The crate-level docs for `egui` (at https://docs.rs/egui generated from [`egui/src/lib.rs`](https://github.com/emilk/egui/blob/master/egui/src/lib.rs)) should be improved with a tutorial. In addition, all major parts of egui should have a quick example in the crate-level docs

* Crate-level:
* [x] intro tutorial
* [ ] FAQ (see https://github.com/emilk/egui/issues and https://github.com/emilk/egui/discussions/categories/q-a for what the common questions are)
* [ ] examples for common widgets, layouts etc (see below)
* Crate-level AND item-level examples for:
* [x] `Label`
* [x] `Hyperlink`
* [x] `TextEdit`
* [x] `Button`
* [x] `CheckBox`
* [x] `RadioButton`
* [ ] `SelectableLabel`
* [x] `Image`
* [ ] `ComboBox`
* [x] `Slider`
* [x] `DragValue`
* [x] `Separator`
* [x] `CollapsingHeader`
* [ ] horizontal and vertcial layouts
* [ ] centered vertical layouts
* [ ] `Grid`
* [ ] tooltips (`on_hover_text`)
* [ ] Add a few gifs to the docs (not too many though, as they are difficult to maintain)
* Links from the web demo to the docs
* [x] Link from the widget gallery to the docs
* [ ] Link from the slider demo to the docs for the slider
* [ ] …
* Links from the docs to the web demo
* [x] Link from the crate-level docs to the web demo
* [ ] Link from the `Slider`-docs to the slider demo window
* [x] Links from the web demo to its own source code

# other
* [x] Meta: make the CI run all the doc tests
* [x] Fix the `eframe` doc build issues: https://github.com/emilk/egui/issues/146
* [ ] Add simple demo of loading and displaying an image
* [ ] [`epi::http` in not showing up in the documentations](https://github.com/emilk/egui/issues/381)

Start with the crate-level documentation in egui/src/lib.rs and review the unchecked items in this tracking issue, including widget examples, layout examples, FAQ content, and web-demo links. Check existing documentation examples and doctests first. Done means completing a clearly scoped subset with examples running as doctests and the relevant documentation or demo links updated.

Written by the indexing model from the issue text.