DOC: pull files via Git LFS on readthedocs hosts

The indicator environment variable is listed at
https://docs.readthedocs.io/en/stable/builds.html

This solution follows the current best practice presented at
readthedocs/readthedocs.org#1846

[ci skip]

Author: slivingston

doc/conf.py

@@ -21,3 +21,10 @@
 html_theme = 'alabaster'
 html_static_path = ['_static']
 htmlhelp_basename = 'hardsharedoc'
+
+
+# Prepare to build on hosts of https://readthedocs.org/
+import os
+if os.environ.get('READTHEDOCS', 'False') == 'True':
+    import subprocess
+    subprocess.check_call('./get-deps.sh')

doc/get-deps.sh

@@ -0,0 +1,26 @@
+#!/bin/sh -e
+# Script to fetch and prepare dependencies to build documentation
+#
+#
+# SCL <[email protected]>
+# Copyright (C) 2020 rerobots, Inc.
+
+
+GLFSVERSION=2.11.0
+FNAME=git-lfs-linux-amd64-v${GLFSVERSION}.tar.gz
+
+
+if [ ! -f $FNAME ]; then
+    curl -L -O https://github.com/git-lfs/git-lfs/releases/download/v${GLFSVERSION}/${FNAME}
+fi
+
+HASHNOW=`sha512sum $FNAME | cut -d ' ' -f1`
+if [ "$HASHNOW" != "ed84947ddf492ef7a9480cb826e2667240743cc10a07e186a5759b0b2961decc176cfb7ede2e21b7e764b200dd0b431297be30597553282ef4da5ce10c2fc4d7" ]; then
+    echo "Error: wrong checksum: $HASHNOW"
+    exit 1
+fi
+
+tar -xzf $FNAME
+./git-lfs install
+./git-lfs fetch
+./git-lfs checkout