Update terminal docs, add Python editor plugin docs.

Author: ntoll

docs/assets/images/pyeditor1.gif

@@ -0,0 +1,73 @@
+# Python editor 
+
+The PyEditor is a core plugin.
+
+!!! warning
+
+    Work on the Python editor is in its early stages. We have made it available
+    in this version of PyScript to give the community an opportunity to play,
+    experiment and provide feedback.
+
+    Future versions of PyScript will include a more refined, robust and perhaps
+    differently behaving version of the Python editor.
+
+If you specify the type of a `<script>` tag as either `py-editor` (for Pyodide)
+or `mpy-editor` (for MicroPython), the plugin creates a visual code editor,
+with code highlighting and a "run" button to execute the editable code
+contained therein in a non-blocking worker.
+
+The interpreter is not loaded onto the page until the run button is clicked. By
+default each editor has its own independent instance of the specified
+interpreter:
+
+```html title="Two editors, one with Pyodide, the other with MicroPython."
+<script type="py-editor">
+  import sys
+  print(sys.version)
+</script>
+<script type="mpy-editor">
+  import sys
+  print(sys.version)
+  a = 42
+  print(a)
+</script>
+```
+
+However, different editors can share the same interpreter if they share the
+same `env` attribute value.
+
+```html title="Two editors sharing the same MicroPython environment."
+<script type="mpy-editor" env="shared">
+  if not 'a' in globals():
+    a = 1
+  else:
+    a += 1
+  print(a)
+</script>
+<script type="mpy-editor" env="shared">
+  # doubled a
+  print(a * 2)
+</script>
+```
+
+The outcome of these code fragments should look something like this:
+
+<img src="../../assets/images/pyeditor1.gif" style="border: 1px solid black; border-radius: 0.2rem; box-shadow: var(--md-shadow-z1);"/>
+
+!!! info
+
+    Notice that the interpreter type, and optional environment name is shown
+    at the top right above the Python editor.
+
+    Hovering over the Python editor reveals the "run" button.
+
+Finally, it is possible to specify a target node into which the code editor is
+created:
+
+```html title="Specify a target for the Python editor."
+<script type="mpy-editor" target="editor">
+  import sys
+  print(sys.version)
+</script>
+<div id="editor"></div> <!-- will eventually contain the Python editor -->
+```

docs/assets/images/pyterm2.gif

@@ -45,10 +45,17 @@ name = input("What is your name? ")
 print(f"Hello, {name}")
 </script>
 ```
+<img src="../../assets/images/pyterm2.gif" style="border: 1px solid black; border-radius: 0.2rem; box-shadow: var(--md-shadow-z1);"/>
 
-Once the Python code has finished you are automatically dropped into the
-Python REPL, like this:
+To use the interactive Python REPL in the terminal, use Python's
+[code](https://docs.python.org/3/library/code.html) module like this:
 
-<img src="../../assets/images/pyterm2.gif" style="border: 1px solid black; border-radius: 0.2rem; box-shadow: var(--md-shadow-z1);"/>
+```python
+import code
+
+code.interact()
+```
 
+The end result should look something like this:
 
+<img src="../../assets/images/pyterm3.gif" style="border: 1px solid black; border-radius: 0.2rem; box-shadow: var(--md-shadow-z1);"/>

docs/assets/images/pyterm3.gif

@@ -75,5 +75,6 @@ nav:
     - Workers: user-guide/workers.md
     - Builtin helpers: user-guide/builtins.md
     - Python terminal: user-guide/terminal.md
+    - Python editor: user-guide/editor.md
     - Plugins: user-guide/plugins.md
     - Example apps: user-guide/examples.md