docs(8.0): remove unbacked Cortex '5.2x' perf claim + dangling /docs/cortex/comparison link

The installation guide asserted a '5.2x geometric mean speedup' linking a /docs/cortex/comparison
page that does not exist (404) and has no backing benchmark — an unbacked performance claim
shipping in the public package, against the evidence rule. Replaced with a qualitative
native-acceleration statement (no number, no dead link) until a measured, reproducible
open-core-vs-native comparison is published. Also dropped the dead 'cortex/comparison' next-link
from PLUGINS.md frontmatter.

docs/PLUGINS.md

@@ -7,7 +7,6 @@ template: guide
 order: 4
 description: Replace any Brainy subsystem — distance functions, embeddings, vector index, metadata index, aggregation — with a custom implementation or optional native acceleration.
 next:
-  - cortex/comparison
   - guides/storage-adapters
 ---
 

docs/guides/installation.md

@@ -58,7 +58,7 @@ const brain = new Brainy({ plugins: ['@soulcraft/cortex'] })
 await brain.init()  // native providers registered during init
 ```
 
-Cortex delivers a **5.2x geometric mean speedup** — see [Brainy vs Cortex](/docs/cortex/comparison) for measured benchmarks.
+Cortex registers native (Rust/SIMD) vector, metadata, and graph engines behind the same Brainy `find()` API — no code changes, an optional dependency for production-scale workloads.
 
 ## Server-only since 8.0