feat: add tests and CI, and remove the module template scaffolding

[engels74]

Summary

Removes the caddy-dns template scaffolding this module still shipped, adds the
tests and CI it had none of, and expands the documentation. Structure follows
caddy-dns/cloudflare.

Template scaffolding

The module still carried the template's placeholder comments, including three
// TODO: This is just an example markers and, on the UnmarshalCaddyfile doc
comment, the line:

**THIS IS JUST AN EXAMPLE AND NEEDS TO BE CUSTOMIZED.**

That doc comment is published verbatim on pkg.go.dev, so it was the first thing
a prospective user read about configuring the module.

Caddyfile parsing

Restructured along the lines of caddy-dns/cloudflare: the directive name is
consumed once rather than in a loop, and the inline and block forms are now
mutually exclusive rather than partially overlapping. Both supported syntaxes
are unchanged from a user's point of view:

tls {
	dns njalla {env.NJALLA_API_TOKEN}
}

tls {
	dns njalla {
		api_token {env.NJALLA_API_TOKEN}
	}
}

Provision now reports an empty API token instead of passing it through. A
placeholder such as {env.NJALLA_API_TOKEN} resolves to an empty string when
the variable is not set in the process running Caddy, which otherwise surfaces
much later as an authentication failure against the Njalla API. This is a
common cause of confusion when Caddy runs under systemd, which does not inherit
the invoking shell's environment.

No token format validation is included. caddy-dns/cloudflare can do this because
Cloudflare's token formats are documented; Njalla publishes no such format, and
guessing a pattern risks rejecting valid tokens.

Tests

The module previously had none. The new tests cover both Caddyfile syntaxes,
the argument and subdirective error paths, placeholder resolution during
provisioning, the empty-token rejection, and module registration.

CI

Three jobs:

• formatting, go vet, and race-enabled tests;
• an xcaddy build that asserts dns.providers.njalla appears in
  caddy list-modules, which a plain package build does not prove;
• a Docker image build.

Docker

Adds a Dockerfile mirroring the one in caddy-dns/cloudflare, building Caddy
with this module via xcaddy. It builds an image locally and CI builds it on
every run so it cannot rot. Publishing to a registry is deliberately not
configured here, since that is an organisation-level decision; caddy-dns/
cloudflare does publish to GHCR on tags, and the same could be added later if
wanted.

Documentation

Adds Caddyfile examples alongside the existing JSON one, including the global
acme_dns form, and covers building with xcaddy or Docker.

Documents Njalla's ACME token option, which grants exactly the permissions the
DNS-01 challenge needs, and adds troubleshooting notes for the two failure modes
reported in this repository: the build error in #5 and the DNS challenge cleanup
failure in #6.

Validation

go vet ./...          clean
go test -race ./...   ok
gofmt -l .            clean

Built with xcaddy against Caddy v2.11.4; dns.providers.njalla registers, and
caddy adapt produces the expected TLS automation policy for both Caddyfile
syntaxes while rejecting an unknown subdirective with a clear message.

Also built with libdns/njalla#6 applied, confirming the updated provider links
and registers correctly.

Follow-up

This PR does not change go.mod. The module currently pins libdns/njalla at a
2025 commit pseudo-version, because that repository has never been tagged. Once
libdns/njalla#6 merges and a release is tagged, a follow-up should bump this
dependency to that version, which is what delivers the DNS challenge cleanup fix
tracked in #6 here.