docs(skills): repoint dead Megatron Core activation recomputation link

The linked page returns 404:
  https://docs.nvidia.com/megatron-core/developer-guide/latest/api-guide/transformer.html#activation-recomputation

This was pre-existing on main but masked: the Sphinx build failed first on
the broken skills link depth, so the linkcheck step never ran. With that
fixed, linkcheck reaches this page and fails the docs build.

Repoint to the api-guide index, verified HTTP 200.

Signed-off-by: Kamran Jafari <kjafarisadeg@nvidia.com>
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: kamran-nvidia

skills/nemo-mbridge-perf-activation-recompute/SKILL.md

@@ -264,4 +264,4 @@ Limitations of this evidence:
 - `skills/nemo-mbridge-perf-memory-tuning/SKILL.md`
 - `skills/nemo-mbridge-perf-cuda-graphs/SKILL.md`
 - `skills/nemo-mbridge-perf-cpu-offloading/SKILL.md`
-- Megatron Core activation recomputation guide: <https://docs.nvidia.com/megatron-core/developer-guide/latest/api-guide/transformer.html#activation-recomputation>
+- Megatron Core activation recomputation guide: <https://docs.nvidia.com/megatron-core/developer-guide/latest/api-guide/index.html>