calc: add config wizard handler and better setting docstring

The docstring for `configure()` is currently used by the website build
process to auto-generate a page describing the core module settings. At
some point we'll figure out how to get away from that, but at present
it's a necessary hack to avoid manually maintaining that page.

sopel/modules/calc.py

@@ -32,7 +32,25 @@ def quote(s):
 class CalcSection(StaticSection):
     oblique_url = ValidatedAttribute('oblique_url',
         default='https://tumbolia-sopel.appspot.com/')
-    """Full URL of the Oblique service instance to use for Python evaluation"""
+    """Full URL of the Oblique service instance to use
+
+    Called for Python evaluation, instead of running unrestricted user input in
+    the local interpreter.
+
+    The trailing slash is required.
+    """
+
+
+def configure(config):
+    """
+    | name | example | purpose |
+    | ---- | ------- | ------- |
+    | oblique\\_url | https://tumbolia-sopel.appspot.com/ | URL to an [Oblique](https://github.com/sopel-irc/oblique) instance |
+    """
+    config.define_section('calc', CalcSection)
+    config.calc.configure_setting('oblique_url',
+                                  "URL of an Oblique instance, including the "
+                                  "trailing slash:")
 
 
 def setup(bot):