[draft] restructured the failover document

[XiShanYongYe-Chang]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

The existing documentation on failover has the following problems and needs to be restructured.

1. Lack of an overall overview page
   After users enter the failover directory, there is no introductory Overview page for guidance. Users are directed straight to detailed technical documents, with no entry point to understand what it is, why it exists, and how to make choices.

2. Sidebar order inconsistent with cognitive logic
   Current order:
   cluster-status-maintenance → cluster-failover → cluster-taint-management → failover-analysis → application-failover

   Problems:

   • cluster-taint-management (Taint is the core mechanism that triggers failover) is placed after cluster-failover, while it is actually a prerequisite for understanding cluster failover.
   • failover-analysis is sandwiched between cluster failover and application failover, leaving readers unclear whether it is an in-depth analysis of cluster failover.
   • cluster-status-maintenance is displayed directly without contextual explanation, so readers cannot figure out its relationship with failover.

3. Content duplication and fragmentation

• StatePreservation is explained in both application-failover.md and cluster-failover.md. The content is presented separately in each document without comparative clarification.
• Feature Gate information is scattered across multiple documents (Failover, GracefulEviction, PropagateDeps, StatefulFailoverInjection). Users have to browse multiple pages to confirm which feature gates need to be enabled.

4. Undifferentiated concepts and operational procedures
   cluster-failover.md mixes multiple layers of content together: background motivation, enabling methods, configuration guidance, and state preservation. The bloated structure makes it inconvenient for quick lookup.

5. Ambiguous positioning of failover-analysis.md
   The title "Analysis" does not clearly indicate that this is an in-depth reference document. As a result, users are unsure whether they need to read it and at which stage it should be reviewed.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

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[gemini-code-assist]

Code Review

This pull request restructures the Karmada failover documentation by introducing a comprehensive overview page and standardizing the presentation of prerequisites and feature gates across both English and Chinese versions. The changes improve the logical flow by linking health monitoring, taint management, and failover internals more effectively. Review feedback suggests refining the feature gate requirements table for accuracy, ensuring consistent formatting in documentation tables, and removing redundant sections in the Chinese translation to maintain alignment with the English source.

[gemini-code-assist]

The Required Feature Gates row in this comparison table is slightly misleading.

1. Application Failover primarily relies on PropagateDeps (to ensure dependencies move with the app) and GracefulEviction (if using Graciously mode).
2. Cluster Failover primarily relies on Failover and GracefulEviction (if using Gracefully mode).

Listing only one gate per column oversimplifies the requirements. It would be better to list the primary gates or refer to the Feature Gates Reference table below for complete details.

Suggested change

	| **Required Feature Gates** | `Failover` (Beta, off by default) | `GracefulEviction` (Beta, on by default) |
	| **Required Feature Gates** | Failover (See reference below) | PropagateDeps & GracefulEviction |

[gemini-code-assist]

For consistency in the Default column, please apply bolding to both Off and On states, or remove it from Off. Currently, only Off is bolded in lines 51 and 54.

Suggested change

	| `Failover` | Beta | **Off** | Required for cluster failover. Enables taint-based workload eviction and rescheduling. |
	| `GracefulEviction` | Beta | On | Required for graceful eviction during both cluster and application failover. Delays removal of old workloads until new cluster is ready. |
	| `PropagateDeps` | Beta | On | Required for application failover. Ensures application dependencies are migrated together. |
	| `StatefulFailoverInjection` | Alpha | **Off** | Required for stateful application failover. Enables state data injection via JSONPath rules. |
	| Failover | Beta | **Off** | Required for cluster failover. Enables taint-based workload eviction and rescheduling. |
	| GracefulEviction | Beta | **On** | Required for graceful eviction during both cluster and application failover. Delays removal of old workloads until new cluster is ready. |
	| PropagateDeps | Beta | **On** | Required for application failover. Ensures application dependencies are migrated together. |
	| StatefulFailoverInjection | Alpha | **Off** | Required for stateful application failover. Enables state data injection via JSONPath rules. |

[gemini-code-assist]

This section "Why Cluster Failover Is Required" (为何需要集群故障迁移) was removed from the top of the file to reduce bloat and move overview content to the new overview.md. However, it was added back at the end of this Chinese version, which is inconsistent with the English version and the PR's stated goal of restructuring. Please remove these lines to maintain consistency.