[doc/use_cases] md format use cases document.
[doc/use_cases] md format use cases document.

Signed-off-by: Miguel Osorio <c7c97b215f812c2e8a32efd57ad5e976a6eb9e45@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan