add link_check target calling linkchecker to manual build makefile

doc/Makefile

@@ -225,6 +225,13 @@ char_check :
 role_check :
 	@( env LC_ALL=C grep -n ' :[a-z]\+`' $(RSTDIR)/*.rst && exit 1 || : )
 
+link_check : $(VENV) html
+	@(\
+		. $(VENV)/bin/activate ; env PYTHONWARNINGS= PYTHONDONTWRITEBYTECODE=1 \
+		linkchecker -F html --check-extern html/Manual.html ;\
+		deactivate ;\
+	)
+
 xmlgen : doxygen/xml/index.xml
 
 doxygen/Doxyfile: doxygen/Doxyfile.in

doc/src/Build_manual.rst

@@ -48,7 +48,7 @@ Build using GNU make
 
 The LAMMPS manual is written in `reStructuredText <rst_>`_ format which
 can be translated to different output format using the `Sphinx
-<https://sphinx-doc.org/>`_ document generator tool.  It also
+<https://www.sphinx-doc.org/>`_ document generator tool.  It also
 incorporates programmer documentation extracted from the LAMMPS C++
 sources through the `Doxygen <https://doxygen.nl/>`_ program.  Currently
 the translation to HTML, PDF (via LaTeX), ePUB (for many e-book readers)
@@ -87,6 +87,7 @@ folder.  The following ``make`` commands are available:
    make anchor_check  # check for duplicate anchor labels
    make style_check   # check for complete and consistent style lists
    make package_check # check for complete and consistent package lists
+   make link_check    # check for broken or outdated URLs
    make spelling      # spell-check the manual
 
 ----------

doc/utils/requirements.txt

@@ -6,3 +6,4 @@ breathe
 Pygments
 six
 pyyaml
+linkchecker