docs/Makefile: Fix doc build outside tox when using make command

When building package documentation outside tox by calling make in the
docs directory, the include of Makefile.sphinx inside the docs Makefile
was failing as its relative path was invalid.

So adapt this relative path according if the SWH_PACKAGE_DOC_TOX_BUILD
environment variable is set or not.

docs/Makefile

+ifdef SWH_PACKAGE_DOC_TOX_BUILD
 include swh-docs/Makefile.sphinx
+else
+include ../../swh-docs/Makefile.sphinx
+endif