Add internal components like `Tab`, `TableCell` etc.. to show on the search options

[ShimiSun]

[spencerkordecki]

@ShimiSun Looks like all that would need to be done to get more components to show up in the search results would be to add them to the object in https://github.com/grommet/grommet-site/blob/master/src/structure.js.

The only problem with this is that the same object is used to populate /components which requires a file for each component in this directory: https://github.com/grommet/grommet-site/tree/master/src/screens

Those items are then used to populate the screen by being pulled in here: https://github.com/grommet/grommet-site/blob/master/src/screens/Components/index.js

Since the docs for all these components appear to all be complete in the main repo, the task would be more about moving items around, but would still be a pretty substantial change. If this were to go through, there would need to be a decision made about how these internal components would be structured in structure.js to make sure they show up in the correct headings and then mockups for the icons to display these components on the /components route.

Let me know if you want any help on this, I'm happy to do some work and open a PR for it.