docs: remove pages no longer exist

Author: windreamer

docs/en/get_started/index.rst

@@ -2,8 +2,7 @@
    :maxdepth: 1
    :caption: Getting Started
 
-   xtuner_v1.md
    installation.md
    sft.md
    mllm_sft.md
-   grpo.md
+   grpo.md

docs/zh_cn/get_started/index.rst

@@ -2,7 +2,6 @@
    :maxdepth: 1
    :caption: 开始使用
 
-   xtuner_v1.md
    installation.md
    sft.md
    mllm_sft.md