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-serve-help.txt

@@ -0,0 +1,29 @@
+$ datasette serve --help
+
+ Usage: datasette serve [OPTIONS] [FILES]...
+
+  Serve up specified SQLite database files with a web UI
+
+Options:
+  -h, --host TEXT           host for server, defaults to 127.0.0.1
+  -p, --port INTEGER        port for server, defaults to 8001
+  --asgi                    Run in ASGI mode
+  --debug                   Enable debug mode - useful for development
+  --reload                  Automatically reload if code change detected -
+                            useful for development
+  --cors                    Enable CORS by serving Access-Control-Allow-
+                            Origin: *
+  --load-extension PATH     Path to a SQLite extension to load
+  --inspect-file TEXT       Path to JSON file created using "datasette
+                            inspect"
+  -m, --metadata FILENAME   Path to JSON file containing license/source
+                            metadata
+  --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
+  --config CONFIG           Set config option using configname:value
+                            datasette.readthedocs.io/en/latest/config.html
+  --version-note TEXT       Additional note to show on /-/versions
+  --help-config             Show available config options
+  --help                    Show this message and exit.