docs: Consolidate repository README.md and docs/index.md

The page `docs/index.md` was copy of the Open IoT SDK repository's top-
level `README.md` with minor improvements. To avoid duplication, update
the top-level `README.md` to bring in those improvements and convert
`docs/index.md` into an alias of `README.md`.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>