tidb-lightning: add deprecation notice to Web Interface page

[alastori]

Summary

• Add a deprecation warning at the top of the Lightning Web Interface docs page
• The Web Interface is deprecated as of v8.5.6 and will be removed in v8.5.7
• Directs users to the tidb-lightning CLI and IMPORT INTO statement as alternatives

Ref pingcap/tidb#67697

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[gemini-code-assist]

Code Review

This pull request adds a deprecation warning for the TiDB Lightning Web Interface, noting its removal in v8.5.7 and recommending the use of the CLI or IMPORT INTO statement instead. Feedback was provided to use more professional terminology in the documentation and to clarify the GitHub issue reference.

[gemini-code-assist]

The term "broken" is too informal for technical documentation. Consider using "unavailable" or "non-functional" instead. Additionally, it is clearer to refer to the GitHub issue as "issue #67697".

Suggested change

> Starting from v8.5.6, the TiDB Lightning Web Interface is deprecated and will be removed in v8.5.7. The web UI build has been broken since v8.4.0. Use the [`tidb-lightning` CLI](/tidb-lightning/tidb-lightning-overview.md) or the [`IMPORT INTO`](/sql-statements/sql-statement-import-into.md) statement instead. If this affects your workflow, comment on [#67697](https://github.com/pingcap/tidb/issues/67697).

> Starting from v8.5.6, the TiDB Lightning Web Interface is deprecated and will be removed in v8.5.7. The web UI has been unavailable since v8.4.0. Use the [`tidb-lightning` CLI](/tidb-lightning/tidb-lightning-overview.md) or the [`IMPORT INTO`](/sql-statements/sql-statement-import-into.md) statement instead. If this affects your workflow, comment on [issue #67697](https://github.com/pingcap/tidb/issues/67697).

References

1. Clarity, simplicity, and readability. Use professional terminology and avoid informal language like 'broken'. ^(link)