Add documentation for the cozystack.installer Ansible collection
that automates full deployment pipeline: OS preparation, k3s
bootstrap, and Cozystack installation. Includes configuration
reference, per-distro playbook examples, and explicit warning
to pin cozystack_chart_version.

Assisted-By: Claude <noreply@anthropic.com>
Signed-off-by: Aleksei Sviridkin <f@lex.la>

Fix prepare playbook references to use local files instead of
non-existent collection playbooks. Use correct ansible-galaxy
collection install command. Add security warning for k3s join
token. Add missing variables to configuration reference.

Assisted-By: Claude <noreply@anthropic.com>
Signed-off-by: Aleksei Sviridkin <f@lex.la>

Download requirements.yml from git since it is excluded from
the packaged collection. List dependency collections explicitly.
Add cozystack_helm_binary and prepare playbook variables
(cozystack_flush_iptables, cozystack_k3s_extra_args) to the
configuration reference. Add k3s releases link for version.

Assisted-By: Claude <noreply@anthropic.com>
Signed-off-by: Aleksei Sviridkin <f@lex.la>

Clarify that helm-diff is installed automatically for idempotent
upgrades. Add commented cozystack_k3s_extra_args example with
tls-san for NAT environments in the inventory sample.

Assisted-By: Claude <noreply@anthropic.com>
Signed-off-by: Aleksei Sviridkin <f@lex.la>