fix: move en playground doc to metrics folder

[frank-zsy]

Description

Move playground doc file into metrics folder for EN.

Resolved issues

None.

Before submitting the PR, please take the following into consideration

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. If you don't have an issue, please create one.
• Prefix your PR title with feat: , fix: , chore: , docs:, or refactor:.
• The description should clearly illustrate what problems it solves.
• Ensure that the commit messages follow our guidelines.
• Resolve merge conflicts (if any).
• Make sure that the current branch is upto date with the main branch.

[frank-zsy]

/preview

[github-actions]

🪧The website for this PR is deployed at https://open-digger.cn/pull_49/