docs: add namingserver registry documentation

[xchy2004]

What is this PR?

This PR adds basic English documentation for using Seata Namingserver as a registry center.

Changes

• Add an overview of Seata Namingserver.
• Add binary package startup instructions.
• Add Docker pull and run examples.
• Add a Docker Compose example.
• Add common Docker environment variables.
• Add basic Seata client and server registry configuration examples.
• Add token retrieval and transaction group mapping examples.

Why this change?

The current English documentation page for Namingserver is a placeholder. This update provides basic usage instructions for users who want to use Namingserver as the registry center.

Related to apache/incubator-seata#7515

Checklist

• Documentation has been added or updated.
• The change is limited to documentation.
• No source code behavior is changed.