Docs: add minimal xcetool page

Author: pont-us

docs/xcetool.md

@@ -0,0 +1,21 @@
+# Running `xcetool`
+
+xcengine provides a command `xcetool` which converts a Jupyter Python notebook
+to a compute engine. A compute engine consists of:
+
+- A docker container image packaging the notebook code, a Python environment,
+  and an [xcube](https://xcube.readthedocs.io/) server component, letting
+  the notebook results be served over multiple supported APIs and explored
+  in the interactive xcube viewer.
+- An accompanying CWL file defining an OGC
+  [Earth Observation Application Package](docs.ogc.org/bp/20-089r1.html) using
+  the container image. This lets your notebook code run as part of a processing
+  workflow on any EOAP platform.
+
+In the conversion process, xcengine tries to maximize convenience for the user
+by requiring as little extra configuration and input as possible. Input
+variables and their types can be defined by tagging a notebook cell
+(similarly to [papermill](https://papermill.readthedocs.io/)), and output
+datasets are automatically extracted from the notebook’s environment.
+Some user configuration is unavoidable, but xcengine automates much of the
+boilerplate required to create an EOAP.