Python docs: provide mechanism for documenting constants.

Python docstrings don't work for variables, so I hacked
a special comment format for helping pydown generate Markdown
text for the README.md for the exported constant KM_PER_AU,
or any other constants I may want to expose in the future.

source/python/README.md

@@ -14,6 +14,7 @@ To get started quickly, here are some [examples](../../demo/python/).
 ## Contents
 
 - [Topic Index](#topics)
+- [Constants](#constants)
 - [Classes](#classes)
 - [Enumerated Types](#enumerations)
 - [Error Types](#errors)
@@ -146,6 +147,18 @@ these are used in function and type names.
 
 ---
 
+<a name="constants"></a>
+## Constants
+
+---
+
+<a name="KM_PER_AU"></a>
+### `KM_PER_AU = 1.4959787069098932e+8`
+
+**The number of kilometers per astronomical unit.**
+
+---
+
 <a name="classes"></a>
 ## Classes
 
@@ -1482,7 +1495,7 @@ The caller may pass `ofdate` as `True` to return coordinates relative to the Ear
 equator at the specified time, or `False` to use the J2000 equator.
 The returned vector has components expressed in astronomical units (AU).
 To convert to kilometers, multiply the `x`, `y`, and `z` values by
-the constant value `KM_PER_AU`.
+the constant value [`KM_PER_AU`](#KM_PER_AU).
 
 | Type | Parameter | Description |
 | --- | --- | --- |

source/python/astronomy.py

@@ -36,7 +36,7 @@ import datetime
 import enum
 import re
 
-KM_PER_AU = 1.4959787069098932e+8
+KM_PER_AU = 1.4959787069098932e+8   #<const> The number of kilometers per astronomical unit.
 
 _CalcMoonCount = 0
 
@@ -3800,7 +3800,7 @@ def ObserverVector(time, observer, ofdate):
 
     The returned vector has components expressed in astronomical units (AU).
     To convert to kilometers, multiply the `x`, `y`, and `z` values by
-    the constant value `KM_PER_AU`.
+    the constant value #KM_PER_AU.
 
     Parameters
     ----------