chore(deps): update dependency backrefs to v6

Author: renovate[bot]

docs/requirements.txt

@@ -4,14 +4,14 @@ babel==2.17.0 \
     --hash=sha256:0c54cffb19f690cdcc52a3b50bcbf71e07a808d1c80d549f2459b9d2cf0afb9d \
     --hash=sha256:4d0b53093fdfb4b21c92b5213dba5a1b23885afa8383709427046b21c366e5f2
     # via mkdocs-material
-backrefs==5.9 \
-    --hash=sha256:6907635edebbe9b2dc3de3a2befff44d74f30a4562adbb8b36f21252ea19c5cf \
-    --hash=sha256:7fdf9771f63e6028d7fee7e0c497c81abda597ea45d6b8f89e8ad76994f5befa \
-    --hash=sha256:808548cb708d66b82ee231f962cb36faaf4f2baab032f2fbb783e9c2fdddaa59 \
-    --hash=sha256:cc37b19fa219e93ff825ed1fed8879e47b4d89aa7a1884860e2db64ccd7c676b \
-    --hash=sha256:db8e8ba0e9de81fcd635f440deab5ae5f2591b54ac1ebe0550a2ca063488cd9f \
-    --hash=sha256:df5e169836cc8acb5e440ebae9aad4bf9d15e226d3bad049cf3f6a5c20cc8dc9 \
-    --hash=sha256:f48ee18f6252b8f5777a22a00a09a85de0ca931658f1dd96d4406a34f3748c60
+backrefs==6.1 \
+    --hash=sha256:13eafbc9ccd5222e9c1f0bec563e6d2a6d21514962f11e7fc79872fd56cbc853 \
+    --hash=sha256:2a2ccb96302337ce61ee4717ceacfbf26ba4efb1d55af86564b8bbaeda39cac1 \
+    --hash=sha256:3bba1749aafe1db9b915f00e0dd166cba613b6f788ffd63060ac3485dc9be231 \
+    --hash=sha256:4c9d3dc1e2e558965202c012304f33d4e0e477e1c103663fd2c3cc9bb18b0d05 \
+    --hash=sha256:a9e99b8a4867852cad177a6430e31b0f6e495d65f8c6c134b68c14c3c95bf4b0 \
+    --hash=sha256:c64698c8d2269343d88947c0735cb4b78745bd3ba590e10313fbf3f78c34da5a \
+    --hash=sha256:e82bba3875ee4430f4de4b6db19429a27275d95a5f3773c57e9e18abc23fd2b7
     # via mkdocs-material
 certifi==2025.8.3 \
     --hash=sha256:e564105f78ded564e3ae7c923924435e1daa7463faeab5bb932bc53ffae63407 \