🐛 Fix singlehtml target uris to be same-document references (#11970)

Before this change, target_uris would be generated as `index.html#foo`. 
This makes it difficult to rename the generated document and can require reloading the document when following a link.

After this change, they are just `#foo`, which avoids the above problems.

CHANGES.rst

@@ -147,6 +147,9 @@ Bugs fixed
 * #12040: HTML Search: Ensure that document titles that are partially-matched by
   the user search query are included in search results.
   Patch by James Addison.
+* #11970: singlehtml builder: make target URIs to be same-document references in
+  the sense of :rfc:`RFC 3986, §4.4 <3986#section-4.4>`, e.g., ``index.html#foo``
+  becomes ``#foo``. Patch by eanorige.
 
 Testing
 -------

sphinx/builders/singlehtml.py

@@ -41,8 +41,7 @@ class SingleFileHTMLBuilder(StandaloneHTMLBuilder):
     def get_target_uri(self, docname: str, typ: str | None = None) -> str:
         if docname in self.env.all_docs:
             # all references are on the same page...
-            return self.config.root_doc + self.out_suffix + \
-                '#document-' + docname
+            return '#document-' + docname
         else:
             # chances are this is a html_additional_page
             return docname + self.out_suffix