fix: translate tagline

[asmgf]

Please do not create a Pull Request without creating an issue first.

What is the purpose of the change

Currently the main page title includes untranslated tagline (官方网站):

This is quite confusing, e.g. when viewing Google search results -- not clear if this is the right page to open.

Brief changelog

Set tagline according to locale environment variable, as described in facebook/docusaurus#4542 (comment)

Verifying this change

npm run build

• Verify build/index.html has translated title.

<title data-rh="true">RocketMQ · Official Website | RocketMQ</title>

• Verify build/zh/index.html has original title.

<title data-rh="true">RocketMQ · 官方网站 | RocketMQ</title>

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