docs+skill: add searxng-search optional skill and documentation

Closes the remaining gaps from PR #11562 that weren't covered by the
core SearXNG integration landed in #20823.

- optional-skills/research/searxng-search/ — installable skill with
  SKILL.md (curl-based usage, category support, Python example) and
  searxng.sh helper script for health checks and instance queries
- website/docs/user-guide/configuration.md — SearXNG added to the
  Web Search Backends section (5 backends, backend table, per-capability
  split config example, correct search-only note)
- website/docs/reference/environment-variables.md — SEARXNG_URL row
- website/docs/reference/optional-skills-catalog.md — searxng-search entry

The core SearXNG code, OPTIONAL_ENV_VARS, hermes tools picker, and tests
were already on main via #20823.  This commit is purely additive docs +
the optional skill scaffold.

Credits from #11562 salvage:
  @w4rum — original _searxng_search structure
  @nathansdev — tools_config.py integration
  @moyomartin — category support and result formatting
  @0xMihai — config/env var approach
  @nicobailon — skill and documentation structure
  @searxng-fan — error handling patterns
  @local-first — self-hosted-first philosophy and docs

optional-skills/research/searxng-search/scripts/searxng.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+# Usage: ./searxng.sh <query> [max_results] [engines]
+# Example: ./searxng.sh "python async" 10 "google,bing"
+
+QUERY="${1:-}"
+MAX="${2:-5}"
+ENGINES="${3:-google,bing}"
+
+if [ -z "$SEARXNG_URL" ]; then
+    echo "Error: SEARXNG_URL is not set"
+    exit 1
+fi
+
+if [ -z "$QUERY" ]; then
+    echo "Usage: $0 <query> [max_results] [engines]"
+    exit 1
+fi
+
+ENCODED_QUERY=$(echo "$QUERY" | sed 's/ /+/g')
+
+curl -s --max-time 10 \
+    "${SEARXNG_URL}/search?q=${ENCODED_QUERY}&format=json&limit=${MAX}&engines=${ENGINES}"