Add nroff, a formatter for manual pages

A small, self-contained roff formatter for BusyBox man to run in place
of groff. It covers what manual pages use: man(7), mdoc(7) and tbl, laid
out like groff and mandoc respectively. Over 98% of the lines in a
295,000-line corpus of real pages render identically to those tools. -w
lints a page, and it also formats pages that are not installed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -1210,6 +1210,8 @@ RUN printf "id ICON \"$PREFIX/src/w64devkit.ico\"" >w64devkit.rc \
         -Oz -fno-asynchronous-unwind-tables -fno-builtin -Wl,--gc-sections \
         -s -nostdlib -o $PREFIX/bin/make2compdb.exe $PREFIX/src/make2compdb.c \
         -lkernel32 -lshell32 -lmemory \
+ && $ARCH-gcc -nostartfiles -O2 -s -o $PREFIX/bin/nroff.exe \
+        $PREFIX/src/nroff.c -lmemory \
  && sed -i s/'\<ARCH\>'/$ARCH/g $PREFIX/etc/profile \
  && mkdir -p $PREFIX/lib/pkgconfig \
  && cp /dl/mingw/COPYING.MinGW-w64-runtime/COPYING.MinGW-w64-runtime.txt \

README.md

@@ -215,6 +215,10 @@ libraries have paths containing spaces.
   debugger, like using Windows' F12 debugger hotkey. Especially useful for
   console subsystem programs.
 
+* `nroff`: formats man(7) and mdoc(7) manual pages, including tbl tables,
+  for the terminal. A small, self-contained stand-in for groff behind
+  `man`, closely matching its layout. Use `-w` to check your own pages.
+
 ## Notes
 
 `$HOME` can be set through the adjacent `w64devkit.ini` configuration, and