Documentation generation failed

**Type of issue**: bug report

**Impact**: unknown

**If the current behavior is a bug, please provide the steps to reproduce the problem:**
```bash
git clone https://github.com/chipsalliance/rocket-chip
cd rocket-chip
git submodule update --init
sbt
```
In the SBT prompt:
```bash
sbt:rocketchip> docs/mdoc
```
Got following errors:
```
[warn] Multiple main classes detected. Run 'show discoveredMainClasses' to see the list
[info] running mdoc.Main --cwd docs
[error] (run-main-0) java.lang.NoClassDefFoundError: scala/tools/nsc/reporters/AbstractReporter
[error] java.lang.NoClassDefFoundError: scala/tools/nsc/reporters/AbstractReporter
[error] at java.base/java.lang.ClassLoader.defineClass1(Native Method)
[error] at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1012)
[error] at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:150)
[error] at java.base/java.net.URLClassLoader.defineClass(URLClassLoader.java:524)
[error] at java.base/java.net.URLClassLoader$1.run(URLClassLoader.java:427)
[error] at java.base/java.net.URLClassLoader$1.run(URLClassLoader.java:421)
[error] at java.base/java.security.AccessController.doPrivileged(AccessController.java:712)
[error] at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:420)
[error] at sbt.internal.ManagedClassLoader.findClass(ManagedClassLoader.java:98)
[error] at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:587)
[error] at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520)
[error] at mdoc.internal.markdown.MarkdownBuilder$.fromClasspath(MarkdownBuilder.scala:92)
[error] at mdoc.internal.cli.Context$.fromOptions(Context.scala:33)
[error] at mdoc.internal.cli.Settings.validate(Settings.scala:231)
[error] Caused by: java.lang.ClassNotFoundException: scala.tools.nsc.reporters.AbstractReporter
[error] at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:445)
[error] at sbt.internal.ManagedClassLoader.findClass(ManagedClassLoader.java:98)
[error] at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:587)
[error] at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520)
[error] at java.base/java.lang.ClassLoader.defineClass1(Native Method)
[error] at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1012)
[error] at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:150)
[error] at java.base/java.net.URLClassLoader.defineClass(URLClassLoader.java:524)
[error] at java.base/java.net.URLClassLoader$1.run(URLClassLoader.java:427)
[error] at java.base/java.net.URLClassLoader$1.run(URLClassLoader.java:421)
[error] at java.base/java.security.AccessController.doPrivileged(AccessController.java:712)
[error] at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:420)
[error] at sbt.internal.ManagedClassLoader.findClass(ManagedClassLoader.java:98)
[error] at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:587)
[error] Nonzero exit code: 1
[error] (docs / Compile / runMain) Nonzero exit code: 1
[error] Total time: 3 s, completed Jan 18, 2023, 10:59:44 AM
```

```bash
sbt:rocketchip> show discoveredMainClasses
[info] * freechips.rocketchip.groundtest.Generator
[info] * freechips.rocketchip.system.Generator
[info] * freechips.rocketchip.unittest.Generator
```

**Please tell us about your environment:**
- `rocket-chip` master branch
```bash
$ sbt --script-version
1.8.2
$ scala -version
Scala code runner version 3.2.2 -- Copyright 2002-2023, LAMP/EPFL

```

Reproduce the failure from a fresh rocket-chip checkout with git submodule update --init, then run docs/mdoc from the SBT prompt. Inspect the docs/mdoc configuration and the reported scala.tools.nsc.reporters.AbstractReporter class-loading failure. Done means documentation generation completes without the NoClassDefFoundError.

Written by the indexing model from the issue text.