fix: use explicit @id for the LPV example heading

`## Linear Parameter-Varying (LPV) identification` in nonlinear.md and the
H1 in examples/lpv.md generated the same slug, so Documenter could not
resolve the cross-reference. Give the example H1 an explicit `@id
lpv-example` anchor and refer to it explicitly from nonlinear.md, leaving
both heading texts intact.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: baggepinnen

docs/src/examples/lpv.md

@@ -1,4 +1,4 @@
-# Linear Parameter-Varying (LPV) identification
+# [Linear Parameter-Varying (LPV) identification](@id lpv-example)
 
 !!! warning "Experimental"
     LPV identification in this package is considered experimental and may

docs/src/nonlinear.md

@@ -269,7 +269,7 @@ ControlSystemIdentification.LPVStateSpace
 ControlSystemIdentification.lpv_warmstart
 ```
 
-See [Linear Parameter-Varying (LPV) identification](@ref) for a worked example.
+See [Linear Parameter-Varying (LPV) identification](@ref lpv-example) for a worked example.
 
 
 # Video tutorials