style: unify subpage and slot naming

[doodlewind]

We use the term "subpage" instead of "sub-page". Naming suggestion from GPT:

Both "subpage" and "sub-page" are commonly used and accepted spellings, and both convey the same meaning.
However, if you are using either of these terms in the context of website design or development, it is worth noting that the convention in web design is to use "subpage" without a hyphen. This is because hyphens are typically reserved for separating words in URLs and can cause confusion or errors in some web programming languages.

/cc @QiShaoXuan @lawvs @joebeijing

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