Substitute usage information into the documentation

This will simplify keeping documentation in sync with the code.

build-docs.sh

@@ -0,0 +1,31 @@
+#! /bin/bash
+
+REMOTE=origin
+
+set -e
+
+if ! [ -f "docs/src/SUMMARY.md" ]; then
+    echo "Run this from the root of the repo"
+    exit 1
+fi
+
+# build the latest version of the mdbook plugin
+cargo build -p taskchampion-cli --features usage-docs --bin usage-docs
+
+# create a worktree of this repo, with the `gh-pages` branch checked out
+if ! [ -d ./docs/tmp ]; then
+    git worktree add docs/tmp gh-pages
+fi
+
+# update the wortree
+(cd docs/tmp && git pull $REMOTE gh-pages)
+
+# remove all files in the worktree and regenerate the book there
+rm -rf docs/tmp/*
+mdbook build docs
+cp -rp docs/book/* docs/tmp
+
+# add everything in the worktree, commit, and push
+(cd docs/tmp && git add -A)
+(cd docs/tmp && git commit -am "update docs")
+(cd docs/tmp && git push $REMOTE gh-pages:gh-pages)