docs: warn that IPv6 examples use reserved addresses (fixes #13552)

The "Create an IPv6 network" and "Use IPv6 for the default bridge
network" sections used RFC 3849 documentation-reserved addresses
(2001:db8::/64) without warning, causing users to copy unusable
config. Added notes matching the existing style in "Dynamic IPv6
subnet allocation", warning about the reserved range and suggesting
ULA subnets from fd00::/8 as a replacement.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: dvdksn

content/manuals/engine/daemon/ipv6.md

@@ -35,6 +35,13 @@ IPv6 is only supported on Docker daemons running on Linux hosts.
            - subnet: 2001:db8::/64
   ```
 
+> [!NOTE]
+>
+> The address `2001:db8::/64` in these examples is
+> [reserved for use in documentation][wikipedia-ipv6-reserved].
+> Replace it with a valid IPv6 network, for example a
+> [Unique Local Address (ULA)][wikipedia-ipv6-ula] subnet from `fd00::/8`.
+
 You can now run containers that attach to the `ip6net` network.
 
 ```console
@@ -74,6 +81,13 @@ The following steps show you how to use IPv6 on the default bridge network.
    }
    ```
 
+   > [!NOTE]
+   >
+   > The address `2001:db8:1::/64` in this example is
+   > [reserved for use in documentation][wikipedia-ipv6-reserved].
+   > Replace it with a valid IPv6 network, for example a
+   > [Unique Local Address (ULA)][wikipedia-ipv6-ula] subnet from `fd00::/8`.
+
    - `ipv6` enables IPv6 networking on the default network.
    - `fixed-cidr-v6` assigns a subnet to the default bridge network,
      enabling dynamic IPv6 address allocation.