Add option for initial warning cell in try examples directive

[steppi]

Closes #142

This PR updates the code for auto generating the notebooks to have the option to add an initial "warning" cell like discussed in #142. This has been added as a configuration option both at the individual directive level, and at the global level with option try_examples_global_warning_text, similar to the existing try_examples_global_button_text etc.

I just looked the json for one of Scikit-Learn's notebooks and copied that formatting there. Basically, wrapping the text supplied in the config with a <div class='alert alert-warning'>. Jupyter is apparently has some css that's aware of this formatting and styles things appropriately.

[steppi]

Thanks @Carreau. Yes, that should definitely be keyword only.

[steppi]

I've rebuilt the SciPy docs at https::/steppi.github.io/scipy against this branch. See https://steppi.github.io/scipy/reference/generated/scipy.integrate.quad.html#scipy.integrate.quad for a specific example.

[Carreau]

Let's get that in. Thanks !

[steppi]

Thanks @Carreau!