bundle IDS citation style as ids.csl and make it the default

Include the IDS citation style (based on the style developed for
ICLC-10 2023 in Mannheim) as resources/ids.csl and pass it to pandoc
unless the document specifies a csl of its own. Drop the now-redundant
remote csl URL from examples/ids.Rmd.

Change-Id: Iaaff300bfb9d9b7ea80471b73de3bf6019fb9f93
diff --git a/README.Rmd b/README.Rmd
index be3d9f4..57e9cfa 100644
--- a/README.Rmd
+++ b/README.Rmd
@@ -489,7 +489,9 @@
 
 ## Bibliography and Appendix Slides
 
-When you cite with `[@key]` and set a `bibliography:` in the yaml header, pandoc/citeproc generates the reference list **at the very end of the document body**. That is fine when the references are your last content:
+When you cite with `[@key]` and set a `bibliography:` in the yaml header, the references are formatted with the bundled IDS citation style by default (based on the style developed for ICLC-10 2023 in Mannheim). To use a different style, specify it as usual with `csl:` in the yaml header.
+
+Pandoc/citeproc generates the reference list **at the very end of the document body**. That is fine when the references are your last content:
 
 ```markdown
 # References