doc: work-around the false unresolved link warning for CSS attribut…

…e selectors

The warning misfires for `[open]` in the following fragment:

    .admonition-follows + blockquote summary:not([open]):not(:hover) {
        opacity: 0.5;
    }

About CSS attribute selectors:
<http://developer.mozilla.org/en-US/docs/Web/CSS/Attribute_selectors>

src/r3/src/common.md

@@ -110,6 +110,15 @@ body.theme-dark span.center img, body.theme-dark center img {
 body.theme-ayu span.center img, body.theme-ayu center img {
     filter: invert(89%) sepia(90%) hue-rotate(180deg);
 }
+
+/* FIXME: rustdoc's intra-doc processor generates a warning when it encounters a
+   bracketed text with an unknown link target. This misfires for CSS attribute
+   selectors. Make rustdoc happy by defining a fake link target. I'm not sure if
+   it's worth reporting.
+
+[open]: #dummy
+
+*/
 </style>
 <script type="application/javascript">
 <!--