docs: use MDX comment syntax in blog placeholder

Fern parses .md pages as MDX, so HTML comments fail to parse and blank
the page. No other docs page uses HTML comments.

Author: shljessie

docs/blog/tensor-parallelism-optimization.md

@@ -4,24 +4,24 @@
 
 ## Background
 
-<!-- Why TP matters: model sizes past single-GPU memory, the compute/communication
-     tradeoff versus FSDP and pipeline parallelism. -->
+{/* Why TP matters: model sizes past single-GPU memory, the compute/communication
+    tradeoff versus FSDP and pipeline parallelism. */}
 
 ## Where tensor parallelism costs performance
 
-<!-- All-gather / reduce-scatter on the critical path, exposed communication,
-     small GEMM tiles at high TP degree, NVLink versus inter-node bandwidth. -->
+{/* All-gather / reduce-scatter on the critical path, exposed communication,
+    small GEMM tiles at high TP degree, NVLink versus inter-node bandwidth. */}
 
 ## Optimizations
 
-<!-- One subsection per optimization, with the mechanism and how to enable it. -->
+{/* One subsection per optimization, with the mechanism and how to enable it. */}
 
 ## Results
 
-<!-- Model, hardware, TP degrees swept, throughput and step-time deltas. -->
+{/* Model, hardware, TP degrees swept, throughput and step-time deltas. */}
 
 ## Reproducing
 
-<!-- Container tag, framework config flags, launch command. -->
+{/* Container tag, framework config flags, launch command. */}
 
 ## Takeaways