add osx env for docs

gstlal/share/conda/envs/base/gstlal-docs-osx-64.yml

+channels:
+  - conda-forge
+dependencies:
+  - 'python > 3.6'
+  - automake
+  - c-compiler
+  - cxx-compiler
+  - doxygen
+  - fftw
+  - fortran-compiler
+# The below gds dependencies have been commented out until an issue with gds on osx-64 can be
+# resolved. See issue for status: https://git.ligo.org/lscsoft/gstlal/-/issues/72
+#  - gds-lsmp
+#  - gds-framexmit
+  - gobject-introspection
+  - graphviz
+  - gsl
+  - gstreamer
+  - gstreamer-orc
+  - gst-python
+  - gst-plugins-base
+  - gst-plugins-good
+  - gwdatafind
+  - 'h5py < 3.0'
+  - lalapps
+  - ldas-tools-framecpp
+  - libframe
+  - libtool
+  - ligo-gracedb
+  - ligo-scald
+  - ligo-segments
+  - make
+  - 'matplotlib-base < 3.0'
+  - myst-parser
+  - nds2-client
+  - 'numpy < 1.20'
+  - pandas
+  - pkg-config
+  - pluggy
+  - pyfftw
+  - 'pygobject < 3.29'
+  - python-avahi
+  - python-confluent-kafka
+  - python-lalburst
+  - python-lalinspiral
+  - python-lalsimulation
+  - python-ligo-lw
+  - scipy
+  - sed
+  - 'sphinx < 3.0'
+  - sphinx_rtd_theme