docs: unmanaged gateway development guide

[shaneutt]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

In a previous discussion as well as a document shared with contributors we pondered and talked about how we might handle "unmanaged gateways", which are Gateway resources that would reference an existing server that would otherwise not be provisioned by or have its lifecycle managed by the Gateway controller implementation itself. Several implementors noted that supporting an unmanaged gateway option was something they would
need ultimately, but we decided it wasn't something that would be easy (or even ultimately correct) to express via the Gateway specification. Given that context and during a community sync it was suggested to start a guide which would provide high level, and example style ideas about how to implement this, if nothing else just so we had some guidance for future implementors, this patch adds such a guide.

Which issue(s) this PR fixes:

Fixes #892

Does this PR introduce a user-facing change?:

NONE

Additional Notes:

I only ended up covering the topics of:

• nomenclature: naming and describing "unmanaged" and "managed" gateways accordingly
• configuration: how one might actually differentiate between unmanaged and managed gateways
• controller strategy: a high level strategy for implementing unmanaged gateway reconcilation logic using reference Services

I know this is very limited at this point, but I wanted to start small to see how this was received. I would invite others who've solved similar problems to continue growing the guide according to their experience.

[howardjohn]

Here is our approach: https://github.com/istio/istio/pull/35876/files

Which I feel like can be generally applicable. I am not a huge fan of requiring an annotation when we already have and addresses field that can serve the same purpose (and effectively does for cloud LBs, if we consider 'managed' == provision a new IP/LB and 'unmanaged' == use a preprovisioned IP/LB).

[k8s-ci-robot]

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[shaneutt]

Given initial feedback I've decided to hold off on this for the moment. I'll take some time to consider other options.