docs: clarify theme config files in code blocks (close #2533)

[peteruithoven]

Fixes: #2533

Summary
Clarify what files config file examples refer to.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

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If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
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To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:
I'm not sure if .vuepress/theme/index.js or themePath/index.js is more appropriate & understandable. I'm Happy to change it to themePath/index.js.

[billyyyyy3320]

Thanks for your help! We also need to update code blocks in zh version. Is that okay to leave it to you?

[peteruithoven]

@billyyyyy3320 I've added paths to the same codeblocks in zh :)