update doc index

[zhyncs]

Motivation

Update the document index so we can access it at https://lmdeploy.readthedocs.io/en/latest/.

Modification

minor

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
3. If the modification has a dependency on downstream projects of a newer version, this PR should be tested with all supported versions of downstream projects.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[lvhan028]

LGTM