docs: tidy attestation types navigation

- Rename the first page from "Attestation types" to "Overview" so the
  group label and first-page title don't duplicate
- Drop per-page icons in the new section (group icon removed; pages
  inherit the default styling)
- Nest the individual system-type pages (currently just SARIF) under
  the catalogue page via a sub-group "Available system types"

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

Author: AlexKantor87

attestation_types/overview.mdx

@@ -1,7 +1,6 @@
 ---
-title: Attestation types
+title: Overview
 description: The three categories of attestation types in Kosli — custom, generic, and system — and when to use each.
-icon: stamp
 ---
 
 An **attestation** records a fact (or a verdict) against a trail or artifact in a Kosli flow. An **attestation type** defines the shape of that record: what data it accepts, how Kosli validates it, and how the resulting tile renders in the UI.

attestation_types/system/catalogue.mdx

@@ -1,7 +1,6 @@
 ---
 title: Available system types
 description: The catalogue of system attestation types currently available in Kosli, with their versions and what each is for.
-icon: list
 ---
 
 System attestation types ship with the Kosli platform — every organization can use any type listed below without setup.

attestation_types/system/overview.mdx

@@ -1,7 +1,6 @@
 ---
 title: System attestation types
 description: How system attestation types work — schema validation, default compliance, and tile rendering — and how they relate to custom and generic types.
-icon: cube
 ---
 
 System attestation types are Kosli-curated attestation types available to every organization. They ship with:

attestation_types/system/sarif.mdx

@@ -1,7 +1,6 @@
 ---
 title: "System type: SARIF"
 description: The SARIF system attestation type — for recording the output of any SARIF v2.1.0 security scanner against a Kosli trail.
-icon: shield-halved
 ---
 
 `system:sarif` is for attesting the output of any tool that produces SARIF v2.1.0 — Checkov, Trivy, Semgrep, Snyk, CodeQL, Bandit, and so on. Kosli parses the SARIF file, records the findings, and renders a severity-breakdown tile showing the tool name, version, and a list of findings with their locations.

config/navigation.json

@@ -33,15 +33,19 @@
         },
         {
           "group": "Attestation types",
-          "icon": "stamp",
           "pages": [
             "attestation_types/overview",
             {
               "group": "System attestation types",
               "pages": [
                 "attestation_types/system/overview",
-                "attestation_types/system/catalogue",
-                "attestation_types/system/sarif"
+                {
+                  "group": "Available system types",
+                  "pages": [
+                    "attestation_types/system/catalogue",
+                    "attestation_types/system/sarif"
+                  ]
+                }
               ]
             }
           ]