[Doc][Misc] Remove obsolete Sphinx toctree from feature guide index (#14979)

### What this PR does / why we need it?

This PR removes the Sphinx-specific `toctree` directive from
`docs/source/user_guide/feature_guide/index.md`. Since the project uses
MkDocs (configured in `mkdocs.yml`) for documentation and navigation,
the Sphinx `toctree` directive is obsolete and redundant.

### Does this PR introduce _any_ user-facing change?

No. Documentation-only updates are not considered user-facing changes.

### How was this patch tested?

Tested by verifying that the documentation builds correctly under MkDocs
and the navigation structure defined in `mkdocs.yml` is unaffected.

- vLLM version: v0.27.1
- vLLM main:
https://github.com/vllm-project/vllm/commit/ba07e4a48fc951300d97eb506217dd530583dea3

Signed-off-by: x60087760 <xiechangxiu3@h-partners.com>
Co-authored-by: x60087760 <xiechangxiu3@h-partners.com>