Docs for datasette publish and package, closes #337

Also introduced a new mechanism for ensuring the --help examples in the
documentation reflect the current output of the --help commands, via a new
update-docs-help.sh script. Closes #336

docs/datasette-package-help.txt

@@ -0,0 +1,25 @@
+$ datasette package --help
+
+ Usage: datasette package [OPTIONS] FILES...
+
+  Package specified SQLite files into a new datasette Docker container
+
+Options:
+  -t, --tag TEXT            Name for the resulting Docker container, can
+                            optionally use name:tag format
+  -m, --metadata FILENAME   Path to JSON file containing metadata to publish
+  --extra-options TEXT      Extra options to pass to datasette serve
+  --branch TEXT             Install datasette from a GitHub branch e.g. master
+  --template-dir DIRECTORY  Path to directory containing custom templates
+  --plugins-dir DIRECTORY   Path to directory containing custom plugins
+  --static STATIC MOUNT     mountpoint:path-to-directory for serving static
+                            files
+  --install TEXT            Additional packages (e.g. plugins) to install
+  --spatialite              Enable SpatialLite extension
+  --version-note TEXT       Additional note to show on /-/versions
+  --title TEXT              Title for metadata
+  --license TEXT            License label for metadata
+  --license_url TEXT        License URL for metadata
+  --source TEXT             Source label for metadata
+  --source_url TEXT         Source URL for metadata
+  --help                    Show this message and exit.