Improve Rustdoc scrape-examples UI

[willcrichton]

This PR combines a few different improvements to the scrape-examples UI. See a live demo here: https://willcrichton.net/misc/scrape-examples/small-first-example/clap/struct.Arg.html

1. The first scraped example now takes up significantly less screen height.

Inserting the first scraped example takes up a lot of vertical screen space. I don't want this addition to overwhelm users, so I decided to reduce the height of the initial example in two ways: (A) the default un-expanded height is reduced from 240px (10 LOC) to 120px (5 LOC), and (B) the link to the example is now positioned over the example instead of atop the example (only on desktop though, not mobile). The changes to scrape-examples.js and rustdoc.css implement this fix.

Here is what an example docblock now looks like:

2. Expanding all docblocks will not expand "More examples".

The "More examples blocks" are huge, so fully expanding everything on the page would take up too much vertical space. The changes to main.js implement this fix. This is tested in scrape-examples-toggle.goml.

3. Examples from binary crates are sorted higher than examples from library crates.

Code that is written as an example of an API is probably better for learning than code that happens to use an API, but isn't intended for pedagogic purposes. Unfortunately Rustc doesn't know whether a particular crate comes from an example target (only Cargo knows this). But we can at least create a proxy that prefers examples from binary crates over library crates, which we know from --crate-type.

This change is implemented by adding a new field bin_crate in Options (see config.rs). An is_bin field has been added to the scraped examples metadata (see scrape_examples.rs). Then the example sorting metric uses is_bin as the first entry of a lexicographic sort on (is_bin, example_size, display_name) (see render/mod.rs).

Note that in the future we can consider adding another flag like --scrape-examples-cargo-target that would pass target information from Cargo into the example metadata. But I'm proposing a less intrusive change for now.

4. The scrape-examples help page has been updated to reflect the latest Cargo interface.

See scrape-examples-help.md.

r? @notriddle

P.S. once this PR and rust-lang/cargo#11450 are merged, then I think the scrape-examples feature is officially ready for deployment on docs.rs!