### What this PR does / why we need it?
- move llms.txt under docs/source and publish it at /llms.txt via
html_extra_path
- rewrite llms.txt to an LLM-friendly link index
- use _sources markdown links and include missing entry points such as
FAQs
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
- vLLM version: v0.16.0
- vLLM main:
15d76f74e2
Signed-off-by: Yikun Jiang <yikunkero@gmail.com>