Add ability to deprecate icons

[korki43]

Icons can be deprecated by giving them the deprecated tag. They won't show up in search results and have a deprecation notice on their page. This is inspired by #843.

When the first icon is deprecated someone would need to recompile bootstrap.min.css because this adds an alert-danger component.

[korki43]

This is just a proposed solution and will definitely need some discussion.

[mdo]

I'd probably suggest a deprecated: true key instead of using a tag. Thoughts?

[korki43]

Changed it to a deprecated: true tag. I also added an example to preview the deprecation notice alert:

https://deploy-preview-1150--bootstrap-icons.netlify.app/icons/~deprecated/