Support building mdoc through Mill on dev

**Type of issue**: feature request

**Impact**: no functional change

**Development Phase**: request

**Other information**

**What is the current behavior?**
Since merging https://github.com/chipsalliance/rocket-chip/commit/0b7fef44f82b35bf289949b3f8fdc139641f8e10, the steps to build mdoc are no longer supported and need a Mill build equivalent to check the mdoc against the latest dev branch.

Review commit 0b7fef44f82b35bf289949b3f8fdc139641f8e10 and the current development-branch build instructions to understand how mdoc was previously built. Identify where a Mill-based equivalent belongs; done means mdoc can be built and checked against the latest dev branch using Mill.

Written by the indexing model from the issue text.