docs: fix dead anchor links in en/zh docs (#2348)

* docs: fix dead anchor links in en/zh docs

- #loop_control -> #loop-control (heading slug uses hyphens)
- #secondary_model -> #secondary-model
- env-vars model section anchors: kimi_model -> kimi-model
- provider credential section anchors: configtoml -> config-toml
- /provider management anchors: point at the renamed heading in each locale
- hooks: point the stale config-files#hooks reference at the local Configuration section
- en files: replace two leftover Chinese anchors with their English targets

* docs: add missing .md extension to themes page links

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Co-authored-by: qer <wbxl2000@outlook.com>