docs(kic): fallback configuration guide

[czeslavo]

Description

Adds a guide explaining the new FallbackConfiguration feature usage.

Part of Kong/kubernetes-ingress-controller#5934.

Testing instructions

Preview link: https://deploy-preview-7463--kongdocs.netlify.app/kubernetes-ingress-controller/unreleased/guides/high-availability/fallback-config/

Checklist

• Review label added
• Conditional version tags added, if applicable.

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[pmalek]

First round of review (without taking this for a spin):

• I'd add section to this doc so that it's easier to follow. Currently this only has Example scenario which doesn't tell the reader what stage of the scenario they're at (am I deploying manifests, testing them, patching them or verifying that fallback config works?)
• I'd dd a separate (named) section which shows how last known config backfill + fallback works on concrete example. This should be separate in my eyes from what already described and the difference should be noted (X route still works, having previous config filled in).

[czeslavo]

@pmalek I noticed that the /debug/config/problems endpoint doesn't work as expected - giving you a heads up so you don't waste time figuring out why it doesn't give you proper responses.

[czeslavo]

First round of review (without taking this for a spin):

• I'd add section to this doc so that it's easier to follow. Currently this only has Example scenario which doesn't tell the reader what stage of the scenario they're at (am I deploying manifests, testing them, patching them or verifying that fallback config works?)
• I'd dd a separate (named) section which shows how last known config backfill + fallback works on concrete example. This should be separate in my eyes from what already described and the difference should be noted (X route still works, having previous config filled in).

Added more granular sections as well as the whole section about backfilling. Thanks for the comments, I hope I addressed all of them. 🙇

[pmalek]

Looks good 👍

Can we already add docs for the feature gate at https://github.com/Kong/docs.konghq.com/blob/86f6865fae98afe729d77583563d982060465578/app/_src/kubernetes-ingress-controller/reference/feature-gates.md ?

[czeslavo]

Can we already add docs for the feature gate at https://github.com/Kong/docs.konghq.com/blob/86f6865fae98afe729d77583563d982060465578/app/_src/kubernetes-ingress-controller/reference/feature-gates.md ?

Updated the feature gates reference (we were missing some from KIC 3.1): 9bbe69b

[mheap]

This is an awesome guide, thank you.

The config debug endpoint isn't working (known issue), but otherwise it all worked as expected (with my latest commit to fix Helm values)

[czeslavo]

Labeling do not merge as we need to:

• decide whether to keep the Mermaid diagram or the PNG one
• update the responses to align with what will be shipped after feat(diag) expose fallback objects kubernetes-ingress-controller#6159 gets merged