feat(notfound): add NotFound component

[dizco]

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Short description of what this resolves:

Redirects users who hit an unknown page to a custom component
Fixes #1410

A couple of things I was not too sure about :

• I've designed it so that only errors within /pages/* are captured by this. I think that for most single page applications, you probably don't want to catch 404's in a global manner (it's ambiguous if the user is not authenticated). I expect most people to have protected their pages directory with a guard of some sort.
• Should this component be within nebular?
• Should we have a direct link in the demo menu to show this page?

[nnixaa]

Hey @dizco, great addition!

• yes, I agree, let's keep it for /pages/* only
• probably not, as this is more of a page rather than component
• yes, let's add a Pages section and this one as a sub-page. Just for demo purposes.

Also, I would suggest a bit of a refactoring - let's move the comonent from @theme/ to the pages/other (didn't think of a better folder name rather than other...) where it belongs better. As this component more of a 'page' component, refferenced in the routes.

[nnixaa]

Also, let's name it NotFoundComponent as the other pages don't have a Page prefix.

[dizco]

@nnixaa what would you think of Miscellaneous instead of Other? Also thought of Extra.

[nnixaa]

Miscellaneous sounds good!

[dizco]

Choice of icons isn't my particular strength, so please feel free to suggest something better if there is haha!

[nnixaa]

@dizco let's add the spaces

[nnixaa]

@dizco cool! Merging now.