docs: add bitprog help

docs/Makefile

@@ -1,24 +1,17 @@
-# Minimal makefile for Sphinx documentation
-#
-
-# You can set these variables from the command line, and also
-# from the environment for the first two.
 SPHINXOPTS ?=
 SPHINXBUILD ?= sphinx-build
 SOURCEDIR = .
 BUILDDIR = _build
+HELPERS = $(BUILDDIR)/hdl2bit $(BUILDDIR)/prj2bit $(BUILDDIR)/bitprog
 
-# Put it first so that "make" without argument is like "make help".
 help:
  @$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
 
 .PHONY: help Makefile
 
-$(BUILDDIR)/hdl2bit $(BUILDDIR)/prj2bit:
+$(HELPERS):
  @mkdir -p $(@D)
  @python3 ../pyfpga/helpers/$(@F).py -h > $@
 
-# Catch-all target: route all unknown targets to Sphinx using the new
-# "make mode" option. $(O) is meant as a shortcut for $(SPHINXOPTS).
-%: Makefile $(BUILDDIR)/hdl2bit $(BUILDDIR)/prj2bit
+%: Makefile $(HELPERS)
  @$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)

docs/helpers.rst

@@ -10,3 +10,8 @@ prj2bit
 -------
 
 .. literalinclude:: _build/prj2bit
+
+bitprog
+-------
+
+.. literalinclude:: _build/bitprog