Validate domain resource and extend DNS record typed fields

docs/resources/domain.md

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+# dynu_domain Resource
+
+Manages a Dynu DNS domain.
+
+## Example Usage
+
+```terraform
+resource "dynu_domain" "example" {
+  name         = "my-test-domain.example"
+  ipv4_address = "203.0.113.10"
+  ttl          = 300
+}
+
+resource "dynu_dns_record" "www" {
+  hostname    = "www.${dynu_domain.example.name}"
+  record_type = "A"
+  content     = "203.0.113.20"
+}
+```
+
+## Import
+
+Import using the numeric Dynu domain ID:
+
+```bash
+terraform import dynu_domain.example 1234
+```
+
+## Attributes
+
+- `id` (Number) Dynu domain ID.
+- `name` (String) Domain name (forces replacement when changed).
+- `ipv4_address` (String) Optional IPv4 address.
+- `ipv6_address` (String) Optional IPv6 address.
+- `ttl` (Number) Optional TTL.
+- `group` (String) Optional group.
+- `state` (String) Computed state from Dynu API.
+- `token` (String, Sensitive) Computed domain token from Dynu API.
+
+## DNS record examples under this domain
+
+```terraform
+resource "dynu_dns_record" "mx" {
+  hostname    = dynu_domain.example.name
+  record_type = "MX"
+  content     = "mail.my-test-domain.example"
+  priority    = 10
+}
+
+resource "dynu_dns_record" "txt_spf" {
+  hostname    = dynu_domain.example.name
+  record_type = "TXT"
+  content     = "v=spf1 include:_spf.example.com ~all"
+}
+
+resource "dynu_dns_record" "cname" {
+  hostname    = "app.${dynu_domain.example.name}"
+  record_type = "CNAME"
+  content     = "target.example.net"
+}
+
+resource "dynu_dns_record" "srv" {
+  hostname    = "_sip._tcp.${dynu_domain.example.name}"
+  record_type = "SRV"
+  content     = "sip.my-test-domain.example"
+  priority    = 10
+  weight      = 5
+  port        = 5060
+}
+```