docs: add redirects on legacy build system pages

Author: sbidoul

.readthedocs-custom-redirects.yml

@@ -13,3 +13,7 @@
   https://docs.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation?tabs=cmd#enable-long-paths-in-windows-10-version-1607-and-later
 /warnings/venv: >-
   https://docs.python.org/3/tutorial/venv.html
+/reference/build-system/pyproject-toml: >-
+  https://pip.pypa.io/en/stable/reference/build-system/
+/reference/build-system/setup-py: >-
+  https://pip.pypa.io/en/stable/reference/build-system/

docs/html/reference/build-system.md

@@ -182,11 +182,11 @@ earlier.
 ```{versionchanged} 23.1
 The legacy interface where pip could invoke `setup.py install`
 in some circumstances was removed,
-in favor of the fallback behavior described below.
+in favor of the fallback behavior described above.
 ```
 
 ```{versionchanged} 25.3
 The legacy interface where pip could invoke `setup.py build_wheel` or
 `setup.py develop` in some circumstances was removed,
-in favor of the fallback behavior described below.
+in favor of the fallback behavior described above.
 ```