docs: fix DNS authentication to use root domain instead of _mcp-registry subdomain (#388)

Fixes #385

Updates DNS authentication documentation to correctly specify that TXT
records should be placed at the root domain (e.g., `example.com`) rather
than the `_mcp-registry` subdomain.

The implementation correctly looks up TXT records at the root domain,
but the documentation incorrectly instructed users to create records at
`_mcp-registry.<domain>`. This mismatch was causing authentication
failures.

## Why use root domain instead of subdomain?

The root domain approach is clearer: if you want to publish to
`com.example/my-server`, you authenticate with `example.com`, not
`_mcp-registry.example.com`.

It also aligns with other TXT DNS standards, e.g. SPF. (That said, there
are other DNS standards that use subdomains e.g. DKIM 🤷)

## Changes
- Updated publish-server.md to remove `_mcp-registry.` prefix from DNS
examples
- Updated CLI commands reference documentation  
- Updated namespacing explanation documentation

docs/explanations/namespacing.md

@@ -22,7 +22,7 @@ Publishing to a namespace requires proving you control the corresponding identit
 - OIDC tokens in GitHub Actions workflows
 
 **Domain namespaces** (`com.company.*`):
-- DNS verification: TXT record at `_mcp-registry.company.com`
+- DNS verification: TXT record at `company.com`
 - HTTP verification: File at `https://company.com/.well-known/mcp-registry-auth`
 
 ## Namespace Scoping