[AIG]added header hierarchy docs

[daisyfaithauma]

Summary

• added header hierarchy docs

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Documentation checklist

• 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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
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Original Link	Updated Link
https://developers.cloudflare.com/ai-gateway/providers/universal/	https://aig-header-hierarchy.cloudflare-docs-7ou.pages.dev/ai-gateway/providers/universal/

[kathayl]

1. i would restructure the headings of this page. on the top right of the page where it says "on this page" i think we should
   a) get rid of "Description" because it's not helpful
   b) make all three "Examples" a sub header and rename it so it's clear what each example is of. ex: WebSockets Example, Hierarchy Example
   c) WebSockets API heading should have a capitalized S in WebSockets (not Websockets)