Remove initialValue and edit description of default in ParamField

[mycho2003]

Documentation changes

In the ParamField element, we claimed to support an initialValue attribute that pre-fills the API Playground, but this wasn't actually supported (see https://github.com/mintlify/mint/pull/3395). This functionality was added to default, and thus the docs are updated to reflect this. Removed initialValue and edited default.

Should review https://github.com/mintlify/mint/pull/3395 first.

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[ethanpalm]

lgtm when the mint PR ships. Thanks for updating the docs!

also for anyone reviewing this PR or Git history in the future: the quickstart edit is accurate (ref: there is no introduction.mdx file, just https://github.com/mintlify/starter/blob/main/index.mdx )