[ECS-Plugin] Document for user

[armistcxy]

What this PR does: Adds user documentation for the ECS plugin (docs-v1.0.x) covering:

• Prerequisites: piped configuration and IAM permissions (deployment role, task execution role, task role)
• Definition files: task definition file and service definition file format, supported fields, and known limitations
• Deployment modes: service (with ELB, service discovery) and standalone task
• Deployment strategy selection: how the plugin automatically chooses between quick sync and pipeline sync based on container image changes
• Pipeline sync stages: ECS_PRIMARY_ROLLOUT, ECS_CANARY_ROLLOUT, ECS_TRAFFIC_ROUTING, ECS_CANARY_CLEAN, ECS_ROLLBACK
• Application live state: sync status, resource tree, resource metadata, and health status
• Plan preview: how diff is computed for ECS applications
• Migration guide from v0: key behavioral changes including the shift to EXTERNAL deployment controller, drift detection via commit hash tag, and rollback behavior changes

Why we need it:

Which issue(s) this PR fixes:

Fixes #6443

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.92%. Comparing base (fdec720) to head (c93da0a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6712      +/-   ##
==========================================
+ Coverage   29.48%   37.92%   +8.44%     
==========================================
  Files         593       61     -532     
  Lines       63440     5331   -58109     
==========================================
- Hits        18706     2022   -16684     
+ Misses      43289     3193   -40096     
+ Partials     1445      116    -1329     

Flag	Coverage Δ
.	?
.-pkg-app-pipedv1-plugin-analysis	32.43% <ø> (ø)
.-pkg-app-pipedv1-plugin-ecs	?
.-pkg-app-pipedv1-plugin-kubernetes	?
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	?
.-pkg-app-pipedv1-plugin-scriptrun	54.83% <ø> (ø)
.-pkg-app-pipedv1-plugin-terraform	?
.-pkg-app-pipedv1-plugin-wait	33.04% <ø> (ø)
.-pkg-app-pipedv1-plugin-waitapproval	52.71% <ø> (ø)
.-pkg-plugin-sdk	50.34% <ø> (ø)
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	?
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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