[Doc][Misc] Add docs for fused_gdn_gating, postprocess_mamba and swiglustep operators (#15080)

### What this PR does / why we need it?
Add triton operator docs for fused_gdn_gating, postprocess_mamba and
swiglustep following the triton operator doc template (#14966).

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
Documentation-only changes, no runtime code changes.


- vLLM main:
https://github.com/vllm-project/vllm/commit/ba07e4a48fc951300d97eb506217dd530583dea3

Signed-off-by: TangPeng <85704592@qq.com>