docs: remove old implementation plans, keep code descriptions

Remove three planning/progress documents that no longer reflect the
codebase:
- rustfoil-implementation-plan.mdx (task breakdown with prompts)
- rustfoil-implementation-spec.mdx (phased plan with GPU roadmap)
- bl-implementation-progress.mdx (progress tracker)

Reorder remaining sidebar positions and update the homepage link to
point to the inviscid solver docs instead of the removed plan.

Made-with: Cursor

Author: aeronauty

docs-site/docs/bl-implementation-progress.mdx

@@ -1,7 +1,7 @@
 ---
 title: Inviscid Solver Implementation
 description: XFOIL-matching inviscid panel method solver in Rust
-sidebar_position: 15
+sidebar_position: 2
 ---
 
 # Inviscid Solver Implementation

docs-site/docs/inviscid-implementation.mdx

@@ -1,7 +1,7 @@
 ---
 title: Inviscid Solver Validation
 description: Comprehensive validation of RustFoil inviscid solver against XFOIL
-sidebar_position: 16
+sidebar_position: 5
 ---
 
 # Inviscid Solver Validation

docs-site/docs/inviscid-validation.mdx

@@ -1,5 +1,5 @@
 ---
-sidebar_position: 5
+sidebar_position: 3
 ---
 
 # Newton Viscous-Inviscid Coupling

docs-site/docs/newton-vi-coupling.mdx

@@ -1,5 +1,5 @@
 ---
-sidebar_position: 5
+sidebar_position: 6
 title: Paneling Validation
 description: Comparison of XFOIL and RustFoil curvature-based paneling (PANGEN)
 ---