Docs: add GPU/TUN, update endpoints & tool refs

Documentation updates across guides and function references:

- Added var_gpu and var_tun configuration entries to CONFIGURATION_REFERENCE (GPU passthrough and TUN/TAP support), including features and prerequisites.
- Fixed repository URLs throughout UNATTENDED_DEPLOYMENTS and examples: replaced community-scripts/ProxmoxVED with community-scripts/ProxmoxVE and updated curl usage to the new paths.
- Added an "Advanced Configuration Variables" table and examples (var_os, var_version, var_gpu, var_tun, var_nesting) to UNATTENDED_DEPLOYMENTS; adjusted sample apps, hostnames, and container mappings in batch examples.
- Switched API endpoints in API_FUNCTIONS_REFERENCE and API_USAGE_EXAMPLES from http://api.community-scripts.org to https://api.community-scripts.org.
- Expanded BUILD_FUNC_FUNCTIONS_REFERENCE with container resource/ID management helper descriptions (validate_container_id, get_valid_container_id, maxkeys_check, get_current_ip, update_motd_ip).
- Large edits to TOOLS_FUNC_FUNCTIONS_REFERENCE: renamed/refactored helper signatures and docs (pkg_install -> install_packages_with_retry, pkg_update -> upgrade_packages_with_retry), added new tooling functions (fetch_and_deploy_gh_release, check_for_gh_release, prepare_repository_setup, verify_tool_version) and updated examples and feature notes.
- Updated vm/README.md to list additional VM scripts (new and reorganized examples).

These are documentation-only changes to clarify configuration options, correct links and endpoints, and expand the reference material for tooling and build helpers.

docs/guides/CONFIGURATION_REFERENCE.md

@@ -539,6 +539,50 @@ var_nesting=0    # Nested containers disabled
 
 ---
 
+### var_gpu
+
+**Type:** Boolean/Toggle
+**Options:** `yes` or `no`
+**Default:** `no`
+**Description:** Enable GPU passthrough for the container.
+
+```bash
+var_gpu=yes      # Enable GPU passthrough (auto-detect)
+var_gpu=no       # Disable GPU passthrough (default)
+```
+
+**Features enabled:**
+- Auto-detects Intel (QuickSync), NVIDIA, and AMD GPUs
+- Passes through `/dev/dri` and render nodes
+- Configures appropriate container permissions
+- Crucial for media servers (Plex, Jellyfin, Immich)
+
+**Prerequisites:**
+- Host drivers installed correctly
+- Hardware present and visible to Proxmox
+- IOMMU enabled (for some configurations)
+
+---
+
+### var_tun
+
+**Type:** Boolean/Toggle
+**Options:** `yes` or `no`
+**Default:** `no`
+**Description:** Enable TUN/TAP device support.
+
+```bash
+var_tun=yes      # Enable TUN/TAP support
+var_tun=no       # Disable TUN/TAP support (default)
+```
+
+**Required for:**
+- VPN software (WireGuard, OpenVPN)
+- Network tunneling (Tailscale, ZeroTier)
+- Custom network bridges
+
+---
+
 ### var_keyctl
 
 **Type:** Boolean (0 or 1)