Support inline <kbd> tag for displaying keyboard shortcuts

[shoogle]

We would like to show keyboard shortcuts such as Ctrl+Z in our documentation.

Of course, it's already possible to show shortcuts in GitBook as plain text like Ctrl+Z, or as inline code like Ctrl+Z or Ctrl+Z. However, these methods don't have the same appearance and accessibility advantages as a dedicated method.

Instead, we would like to use HTML's dedicated <kbd> tag, which is supported in GitHub Markdown (try it in a comment below!).

Markdown	GitHub rendering
<kbd>Ctrl</kbd>+<kbd>Z</kbd>	Ctrl+Z

Unfortunately, GitBook doesn't currently support the <kbd> tag even when it's manually added to Markdown files via GitHub Sync.

Please add support for the <kbd> tag in GitBook, and apply the necessary CSS styling to make it look nice, like Ctrl+Z. This means a different font compared to plain text, a grey backgound, and shading so it looks like an actual keyboard key.

Note: On GitHub, the <kdb> tag renders as Ctrl+Z, which is quite similar to its inline code rendering: Ctrl+Z.