[bazel] added Bazel getting started to docs
[bazel] added Bazel getting started to docs

Added instructions to get started building software with Bazel to
publicize the work flow to build the Mask ROM and other targets it
depends upon.

Signed-off-by: Drew Macrae <32d48ac45113625c0a04afe7343eae478a303684@google.com>

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