docs: Consolidate info for testing in a dedicated page

The page `docs/README.md` was not added to the website, because we
used `docs/index.md` instead which is an alias of the root-level
`README.md`.

Since `docs/README.md` still contained some useful information for
integration tests, this commit

* renames `docs/README.md` to `docs/testing.md`, and makes it availabe
  in the website
* removes from `docs/testing.md` any information that's not related to
  testing, because non-testing information is already better covered by
  the root-level `README.md`
* moves information about unit tests from the root-level `README` into
  `docs/testing.md`, to consolidate all testing info in one page

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