docs: use internet archive for isabelle.systems

The isabelle.systems site appears to be down for a while, use archive to
stop the constant PR links failures while we think of a longer-term
solution.

Signed-off-by: Rafal Kolanski <rafal.kolanski@proofcraft.systems>

docs/conventions.md

@@ -41,7 +41,7 @@ correct material in here!
     circumstances will be transformed into `?x2.0` which is harder to work with),
     but don't overuse `'`, i.e. `x'''` is not a good name.
 
-[1]: https://isabelle.systems/conventions/naming.html
+[1]: https://web.archive.org/web/20220810201813/https://isabelle.systems/conventions/naming.html
 
 
 ## Directories