docs: address Login Flow v2 review feedback

Fix issues raised by reviewer on PR #743:

- troubleshooting.md: renumber "Getting Help" steps (4→3, 5→4) after
  earlier consolidation left a gap
- installation.md: drop stale "OIDC app" prerequisite; admin access is
  now optional under Login Flow v2 (works on stock Nextcloud 16+)
- semantic-search-architecture.md: rename VECTOR_SYNC_ENABLED to
  ENABLE_SEMANTIC_SEARCH in the Status callout (renamed in v0.58.0)
- configuration.md: remove Quick Start references to deprecated
  oauth-multi-user / oauth-advanced templates and point to
  login-flow-v2.md; update "OAuth, Multi-User BasicAuth" label to
  "Login Flow v2, Multi-User BasicAuth"
- auth-flows.md: fix background-sync diagram so Encrypt+persist step
  no longer crosses into the Nextcloud column

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

docs/troubleshooting.md

@@ -423,7 +423,7 @@ curl -u "$NEXTCLOUD_USERNAME:$NEXTCLOUD_PASSWORD" \
 
 For Login Flow v2 mode, see [Login Flow v2 troubleshooting](login-flow-v2.md#troubleshooting).
 
-### 4. Check Versions
+### 3. Check Versions
 
 ```bash
 # MCP Server version
@@ -435,7 +435,7 @@ python3 --version
 # Nextcloud version (check in admin panel)
 ```
 
-### 5. Open an Issue
+### 4. Open an Issue
 
 If problems persist, open an issue on the [GitHub repository](https://github.com/cbcoutinho/nextcloud-mcp-server/issues) with: