Build Mir and invoke spelling check on built docs

doc/sphinx/Makefile

@@ -36,6 +36,11 @@ full-help: $(VENVDIR)
 	@echo "\n\033[1;31mNOTE: This help texts shows unsupported targets!\033[0m"
 	@echo "Run 'make help' to see supported targets."
 
+build_mir:
+	cd ../../; \
+	cmake -B .build_doc; \
+	cmake --build .build_doc --target doc;
+
 # Shouldn't assume that venv is available on Ubuntu by default; discussion here:
 # https://bugs.launchpad.net/ubuntu/+source/python3.4/+bug/1290847
 $(SPHINXDIR)/requirements.txt:
@@ -72,7 +77,7 @@ run: install
 
 # Doesn't depend on $(BUILDDIR) to rebuild properly at every run.
 html: install
-	. $(VENV); $(SPHINXBUILD) -b dirhtml "$(SOURCEDIR)" "$(BUILDDIR)" -w $(SPHINXDIR)/warnings.txt $(SPHINXOPTS)
+	. $(VENV); cd ../../.build_doc/doc/sphinx; $(SPHINXBUILD) -b dirhtml "$(SOURCEDIR)" "$(BUILDDIR)" -w $(SPHINXDIR)/warnings.txt $(SPHINXOPTS)
 
 epub: install
 	. $(VENV); $(SPHINXBUILD) -b epub "$(SOURCEDIR)" "$(BUILDDIR)" -w $(SPHINXDIR)/warnings.txt $(SPHINXOPTS)
@@ -90,8 +95,8 @@ clean-doc:
 	git clean -fx "$(BUILDDIR)"
 	rm -rf $(SPHINXDIR)/.doctrees
 
-spelling: html
-	. $(VENV) ; python3 -m pyspelling -c $(SPHINXDIR)/spellingcheck.yaml -j $(shell nproc)
+spelling: build_mir html
+	. $(VENV) ; cd ../../.build_doc/doc/sphinx/; python3 -m pyspelling -c $(SPHINXDIR)/spellingcheck.yaml -j $(shell nproc)
 
 linkcheck: install
 	. $(VENV) ; $(SPHINXBUILD) -b linkcheck "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS)