Doc updates (#1522)

* Updates

* Remove preview texts

* Update README.md

* Updates

* Update README.md

* Update README.md

* Minor wording update

* Update README.md

* Update doc on CUDA version

* revert update

* Update readme for issue #1558

* Clean up example section

* Cosmetic updates

- Add a index of build instructions for browsability
- Update build CUDA version from 9.1 to 10

* Fix broken link

* Update README to reflect upgrade to pip requirement

* Update CuDNN version for Linux Python packages

* Clean up content

Updated ordering and add table of contents

* Minor format fixes

* Move Android NNAPI under EP section

* Add link to operator support documentation

* Fix typo

* typo fix

* remove todo section