[doc] Update OpenOCD instructions for Verilator
[doc] Update OpenOCD instructions for Verilator

Adds a link to the OpenOCD installation instructions and removes
mention of the compliance test as whilst it is still present in OpenOCD
it is incomplete and unsupported
(https://github.com/riscv/riscv-openocd/issues/462#issuecomment-606155696)

Signed-off-by: Greg Chadwick <3e4c3d6d87c017d1a2fa161477653b4039c6b2fb@lowrisc.org>

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