docs: Move AUGUR_DEBUG definition to our env vars page…

…instead of being buried in the developer API pages.  Much more
discoverable.

Modifies formatting of the rST to match the new document's existing
formatting, but the content is the same.

Thanks to @joverlee521 for catching this misplacement in post-merge
review.¹

¹ <#1577 (comment)>

augur/debug.py

@@ -1,11 +1,7 @@
 """
 Debug flags and utilities.
 
-.. envvar:: AUGUR_DEBUG
-
-    Set to a truthy value (e.g. 1) to print more information about (handled)
-    errors.  For example, when this is not set or falsey, stack traces and
-    parent exceptions in an exception chain are omitted from handled errors.
+See also :envvar:`AUGUR_DEBUG`.
 """
 from os import environ
 

docs/usage/envvars.rst

@@ -5,6 +5,13 @@ Environment variables
 Augur's behaviour can be globally modified by the values of some specific environment variables.
 These can be especially useful in the context of an entire pipeline or workflow which uses Augur, as the environment variables can be set once for all Augur commands at the start of the pipeline.
 
+.. envvar:: AUGUR_DEBUG
+
+    Boolean.
+    Set to a truthy value (e.g. 1) to print more information about (handled) errors.
+
+    For example, when this is not set or falsey, stack traces and parent exceptions in an exception chain are omitted from handled errors.
+
 .. envvar:: AUGUR_MINIFY_JSON
 
     Boolean.