Update GEP overview to clarify the GEP process

[youngnick]

/kind documentation

What this PR does / why we need it:
This updates the GEP overview page to add a state diagram and clarify some steps.'

Also includes cleaning out the site-src/geps directory because I was having problems getting it to update properly.

Does this PR introduce a user-facing change?:

NONE

[shaneutt]

I think the overview of what the process should roughly be is good, and then specifically asking for GEP issues is good too, so we can better track these on our project boards and for milestones.

/lgtm

I think this should be pretty non-controversial, but I'll put a hold so we can have a couple other people give it a glance before we merge it.

/hold

[shaneutt]

/test all

[mlavacca]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mlavacca, shaneutt, youngnick

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