Add Thai README

[ChaiyoKung]

Description

• Added a new file: docs/th/README-th.md, which is a Thai translation of the original English README.
• Covers all sections: project introduction, features, installation, API examples, I18n usage, plugin usage, sponsors, contributors, and license.
• Preserves Markdown formatting, links, images, and code samples as in the original.

Motivation

To help Thai users better understand and use Day.js.

Related Issue

Helps with #171

Checklist

• Translated README.md to Thai
• Created README-th.md at docs/th/README-th.md with translated content
• Verified completeness and formatting of README-th.md