move content-management and react files left behind in the packages f…

…older (elastic#206874)

## Summary

While looking at the `packages` folder at the root of Kibana, I noticed
some files were left over in otherwise empty folders:
- 2 README files were left in the `content-management` folder
- 1 README file and 1 png file were left in the `react` folder

The rest of the content was moved to a new location as part of the
Sustainable Kibana Architecture effort (see [this
PR](elastic#205593) and [that
one](elastic#205924)) and I wonder if
those few files were left behind by mistake.

I did not making any changes to the content of the files, I just moved
them to their respective new locations.

Please let me know if these were left behind intentionally, or if they
should be deleted instead of moved!

### Notes

The `appex-sharedux` codeowner only appeared after pushing the second
commit which impacts the `react` folder. I realized that the codeowners
file was pointing to the folder within
`src/platform/packages/shared/content-management/content_insights` and
`src/platform/packages/shared/content-management/favorites` so update it
to point to the parent folder, which now contains the moved README
files. I hope that's ok!

---------

Co-authored-by: kibanamachine <[email protected]>