[Docs] add and publish llms.txt for LLM discovery (#6886)

### 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>

docs/source/conf.py

@@ -125,6 +125,8 @@ html_theme_options = {
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
 # html_static_path = ['_static']
+# Copy llms.txt to site root so it is available as /llms.txt.
+html_extra_path = ["llms.txt"]
 
 READTHEDOCS_VERSION_TYPE = os.environ.get("READTHEDOCS_VERSION_TYPE")
 if READTHEDOCS_VERSION_TYPE == "tag":