Docs: add indeterminate disabled checkbox example

[julien-deramond]

Based on the work of @louismaximepiton this PR proposes to add a "Disabled indeterminate checkbox" example in Forms > Check & radios > Disabled.

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[julien-deramond]

The other way around would be to set checkbox.indeterminate = true only to the first example in .bd-example-indeterminate:

document.querySelectorAll('.bd-example-indeterminate')
  .forEach(example => {
    example.querySelector('[type="checkbox"]').indeterminate = true
  })

IDK what's the preferred approach here.

[mdo]

Would we ever want to add a custom indeterminate attribute to use as a selector? If not, I think a class might make most sense.

[julien-deramond]

Would we ever want to add a custom indeterminate attribute to use as a selector? If not, I think a class might make most sense.

You mean in the library itself (by having let's say as an example an .indeterminate class that will automatically launch the JS and then remove the class) or just for the documentation?

For the documentation I tried to avoid having an extra-class so that it is still understandable for the user (when copy/pasting or editing via StackBlitz) that the indeterminate state can only be set by JavaScript.

[mdo]

That all sounds good. I appreciate the goal of keeping the source code lean for the end user.