add backend protocol to implementers guide

[joerober]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
Adds section on backend protocol selection to implementors guide explaining that backend protocol selection exists with link to user guide and link to supporting protocols in GEP-1911.

Which issue(s) this PR fixes:

Fixes #2453

Does this PR introduce a user-facing change?:

NONE

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: joerober / name: Joel Roberts (48ab57a)

[k8s-ci-robot]

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[dprotaso]

/ok-to-test

[dprotaso]

/lgtm

[dprotaso]

/assign @youngnick @shaneutt @robscott

I believe this is the last bit for the backend protocol selection GEP.

[youngnick]

/retest

[robscott]

Thanks @joerober!

[robscott]

Nit: It would be good to reword this to focus on the context that this is the Implementer's Guide, and thus intended for implementers to read. What do implementers need to know about this feature? I think the focus in this guide should be to talk about how you might implement it, what this concept usually maps to in common underlying dataplanes like Envoy or NGINX, and where implementers can find more information (I think you already have that part in "References", but it might be good to have more specifics such as the well defined protocols that we recommend implementations support. It would also be good to include keywords such as websockets and HTTP/2 for implementers that may be looking for how to support these concepts with their Gateway API implementation.