Add codespell integration, which checks all sources

Add a make target for running `codespell` on all sources.
Skip running codespell on the build directory (we have javascript etc.
in there).
Only emit warnings but do not fail on spelling errors in diagrams.

Signed-off-by: David Runge <dave@sleepmap.de>

book/.codespellrc

@@ -0,0 +1,5 @@
+# SPDX-FileCopyrightText: 2023 The "Notes on OpenPGP" project
+# SPDX-License-Identifier: CC0-1.0
+
+[codespell]
+skip = ./build,./source/diag/*.svg

book/Makefile

@@ -6,9 +6,11 @@
 
 # You can set these variables from the command line, and also
 # from the environment for the first two.
+CODESPELL ?= codespell
 SPHINXOPTS    ?= -W
 SPHINXBUILD   ?= sphinx-build
 LYCHEE ?= lychee
+PRINTF ?= printf
 SOURCEDIR     = source
 BUILDDIR      = build
 
@@ -19,6 +21,14 @@ help:
 html-linkcheck: clean html
 	@$(LYCHEE) "$(BUILDDIR)/html/"*.html
 
+# spell check all sources
+# NOTE: diagrams are not yet spell checked, but we emit the required changes
+codespell:
+	@$(PRINTF) "The following change suggestions are only warnings!\n"
+	@$(CODESPELL) source/diag || true
+	@$(PRINTF) "The following change suggestions are errors!\n"
+	@$(CODESPELL) .
+
 .PHONY: help Makefile
 
 # Catch-all target: route all unknown targets to Sphinx using the new