docs: add end-to-end ingress setup guide

[Aaravanand00]

This PR adds a step-by-step End-to-End Ingress Setup guide to the documentation site.

The guide walks through deploying a sample application, configuring a Gateway and HTTPRoute, enabling path-based routing, and securing traffic with TLS — all within a single namespace model.

It follows existing documentation structure and shortcodes, and does not introduce any runtime changes.

[Aaravanand00]

Hi @npolshakova mam,

Following your earlier feedback, I’ve moved the End-to-End Ingress guide to the kgateway.dev repository and aligned it with the existing documentation structure (frontmatter, shortcodes, sidebar positioning, and relative links).

The guide demonstrates a single-namespace ingress setup covering Gateway, HTTPRoute, path-based routing, and TLS, and is focused on onboarding clarity.

This continues the work originally proposed in kgateway-dev/kgateway#13399, now submitted in the correct repository.

Would appreciate your review and any suggestions for further improvements.