docs: missing link to apisix_consumer pages

[sakazuki]

Current State

In the menu of References and Concepts, there are no links to APISIX Consumer pages (following pages).

https://apisix.apache.org/docs/ingress-controller/references/apisix_consumer_v2/
https://apisix.apache.org/docs/ingress-controller/concepts/apisix_consumer/

Desired State

Add the links to them in the menu

[sreeram-venkitesh]

Opened #2304 to add the links. I believe this is the only place where we need to add these. I couldn't find any documentation on running the website locally in CONTRIBUTING.md. I'd be happy to add these instructions if it is not already present somewhere else.