docs: Add ECS canary and blue-green deployment example pages

[Prachi-Gupta2808]

What this PR does:
Adds two new example pages to the documentation site: "Canary deployment for ECS app" and "BlueGreen deployment for ECS app". Both pages follow the same format as the existing Kubernetes example pages. The TaskSet mechanics explained on these pages are based on the "Method B: Using Three TaskSets" approach described in this ECS External Deployment guide, which was referenced in the issue as the recommended source for this work.

Changes are added to both docs-dev and docs-v0.57.x:

• docs/content/en/docs-dev/user-guide/examples/ecs-app-canary.md
• docs/content/en/docs-dev/user-guide/examples/ecs-app-bluegreen.md
• docs/content/en/docs-v0.57.x/user-guide/examples/ecs-app-canary.md
• docs/content/en/docs-v0.57.x/user-guide/examples/ecs-app-bluegreen.md

Why we need it:
ECS canary and blue-green deployments rely on a less obvious mechanism than the Kubernetes examples, PipeCD manages up to three ECS TaskSets at once (current, canary, and new) and shifts traffic between ALB target groups rather than doing a native rolling update. A dedicated walkthrough helps users understand what's actually happening at each pipeline stage before they try it themselves.

Which issue(s) this PR fixes:
Part of #4467 (adds the ECS Canary and Blue/Green pages; I would like to do Lambda examples separately after this one is reviewed and merged).

Does this PR introduce a user-facing change?:

• How are users affected by this change: This is a docs-only addition. Users now have two new example/reference pages under User Guide/Examples.
• Is this breaking change: No.
• How to migrate (if breaking change): N/A.

[github-actions]

👋 Hi @Prachi-Gupta2808, welcome to PipeCD and thanks for opening your first pull request!

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[Prachi-Gupta2808]

I would also like to ask, after the review of the text material, if there is any particular format of diagram, flowchart, or screenshot that must be added in the docs. I saw these are added in the already existing example guides and make for a smooth user experience, if so, let me know and I can work on those and add them too.
Thank you.