minor #11066 Add missing index entry for web_link.rst (dunglas)

This PR was submitted for the 4.2 branch but it was merged into the 3.4 branch instead (closes #11066).

Discussion
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Add missing index entry for web_link.rst

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Commits
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3162df8 Add missing index entry for web_link.rst

Author: javiereguiluz

web_link.rst

@@ -1,3 +1,6 @@
+.. index::
+   single: Web Link
+
 Asset Preloading and Resource Hints with HTTP/2 and WebLink
 ===========================================================