fix: update LXD docs URL in firewall warning messages and tests

[Copilot]

LXD documentation moved from documentation.ubuntu.com/lxd/ to canonical.com/lxd/docs/. The firewallDocLink constant and its inline comment in firewall.go still referenced the old domain, causing three tests in firewall_test.go to fail as they matched against .*documentation.ubuntu.com.*.

Changes:

• internal/workshop/lxd/firewall.go: update firewallDocLink constant and comment to https://canonical.com/lxd/docs/latest/howto/network_bridge_firewalld/
• internal/workshop/lxd/firewall_test.go: update URL regex assertions in TestBridgeBlockedWarningDocker, TestBridgeBlockedWarningUFW, and TestBridgeBlockedWarningUnknown to match .*canonical.com/lxd/docs.*

Self-review quick check

• Make decisions that cost a lot to reverse explicit in the PR description.
• Avoid nested conditions.
• Delete dead code and redundant comments.
• Normalise symmetries by sticking to doing identical things identically.

// one way to handle errors
if err := f(); err != nil {
   ...
}

// one way to handle multiple returns
val, err := f()
if err != nil {
   ...
}
...

• Check that coupled code elements, files, and directories are adjacent. For example, test data is stored as close as possible to a test.
• Put variable declaration and initialisation together.
• Divide large expressions into digestable and self-explanatory ones. Use multiple variables if required.
• Put a blank line between two logically different chunks of code.
• Follow the style guide for new error messages.

Docs

• I confirm the PR has no implications for documentation.