kbd role should be a literal node

[agoose77]

I think my original implementation of the kbd role was mistaken to use a parent node. Instead, I think we should behave like a literal e.g. like chemicalFormula, inlineMath, etc.

[rowanc1]

We did this so we can support this:

<p>
  You can also create a new document using the
  <kbd><kbd>Ctrl</kbd>+<kbd>N</kbd></kbd> keyboard shortcut.
</p>

Which is in the MDN docs. Style could be improved, but that works (demo)!

[choldgraf]

I believe the way that sphinx distinguishes these is with the {guilabel} role, which is similar to kbd but doesn't do the "separating out" that {kbd} does (so {kbd}`a + b` becomes a separate a and b in the output)

https://www.sphinx-doc.org/en/master/usage/restructuredtext/roles.html#role-guilabel

Personally I have never been a fan of the naming because it's unnecessarily long to type for a role :-D