align API Gateway definition with CNCF Glossary

[craigbox]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
This site offers a comparison of an API Gateway with the Gateway API, with a very broad definition, which defines almost any ingress system or edge router as an API gateway, regardless of whether the traffic passing through it is user- or machine-facing.

The industry tends towards a more narrow definition, which is well-captured by the CNCF Glossary. I've copied that language into the index and FAQ pages and added a link to the glossary.

Does this PR introduce a user-facing change?:

Updates comparison of Gateway API and API Gateway to use the CNCF's definition of the latter.

[craigbox]

/test pull-gateway-api-verify

[robscott]

Thanks @craigbox! Great cleanup for our docs

[robscott]

Nit: This doesn't really feel like a subheading of "Use Cases" (###) or "Why does a role-oriented API matter?" (##). I know it feels like overkill for this to be H2 (##), but I'm not really sure that it fits very neatly in any of our existing sections.

[craigbox]

I am 99% sure that's a typo

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: craigbox, robscott

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[robscott]

Thanks @craigbox!

/lgtm