doc: fix broken doc links

The project's README.rst references the support board docs with an URL
that's not working these days (see
https://github.com/zephyrproject-rtos/infrastructure/issues/134

) so fix
that URL reference.  While looking for other similar linking cases, I
found a hard URL references that should be using :ref: role, and a
release notes reference to a (now) broken link (fixing that in the
/latest/ version of the 1.10 release notes).

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>