Update get-started.mdx

[kathayl]

be more clear that using AI Gateway requires bringing your own api key for each provider

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• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[kathayl]

@daisyfaithauma - objective of this PR is to be clear that using AI gateway requires bringing your own api key

i've updated the get started page, but we should add to each provider page as well.

ask: can you update this in each provider page (maybe in the prereqs section)

could be something like:
Prerequisites
To use DeepSeek with AI Gateway, ensure you have the following:

-Your AI Gateway Account ID and gateway name.
-An active DeepSeek account with an API token.
-The name of the DeepSeek model you want to use.

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Original Link	Updated Link
https://developers.cloudflare.com/ai-gateway/get-started/	https://kathayl-patch-5.cloudflare-docs-7ou.pages.dev/ai-gateway/get-started/