doc: Update Nuclei custom bf16 extension documentation

[dongyasun]

• The RVV intrinsic documentation supplement the vmv instruction.
• The _rvv-intrinsic-doc link has been updated from v1.0-rec7 to v1.0-ratified.
• Added vector bf16 extension Xxlvfbf to LLVM extension support.

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Important

Update Nuclei bf16 documentation and add vector bf16 extension to LLVM support.

• Documentation Updates:
  • Add vmv.v to vector floating-point move instructions in nuclei_bf16.rst.
  • Update _rvv-intrinsic-doc link from v1.0-rec7 to v1.0-ratified in nuclei_bf16.rst.
• LLVM Extension Support:
  • Add Xxlvfbf to Nuclei custom BFloat16 Extensions in intro.rst.

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