[Doc][Setup] Change python version and vllm version

[ZeldaHuang]

1. Change vllm to 0.6.3.post1 in docs.
2. Remove python 3.8 support(during to ray nightly version requirement)

[github-actions]

prefill	p25	p50	p75	p95	p99	mean
latency(ms)	86.21	1290.97	2610.85	88514.85	131116.89	11402.62

decode	p25	p50	p75	p95	p99	mean
latency(ms)	71.49	106.72	144.06	1086.99	1947.14	203.48

[github-actions]

migration_size	8.00 MB	16.00 MB	24.00 MB	32.00 MB	40.00 MB	48.00 MB	56.00 MB	64.00 MB	72.00 MB	80.00 MB	88.00 MB	96.00 MB	104.00 MB	112.00 MB	120.00 MB	128.00 MB	136.00 MB	144.00 MB	152.00 MB	160.00 MB	168.00 MB	176.00 MB	184.00 MB	192.00 MB	200.00 MB	208.00 MB	216.00 MB	240.00 MB	248.00 MB	272.00 MB	280.00 MB	288.00 MB	304.00 MB	312.00 MB	320.00 MB	424.00 MB	472.00 MB
rayrpc_speed(GB/s)	0.87	1.34	1.63	1.81	1.93	2.00	2.13	2.21	2.24	2.29	2.30	2.38	2.30	2.39	2.35	2.31	2.39	2.32	2.52	2.47	2.61	2.58	2.76	2.68	2.60	2.57	2.68	2.56	2.47	2.78	2.92	2.72	2.72	2.88	3.00	3.12	3.09

migration_size	8.00 MB	16.00 MB	24.00 MB	32.00 MB	40.00 MB	48.00 MB	56.00 MB	64.00 MB	72.00 MB	80.00 MB	88.00 MB	96.00 MB	104.00 MB	112.00 MB	120.00 MB	128.00 MB	136.00 MB	144.00 MB	152.00 MB	160.00 MB	168.00 MB	176.00 MB	184.00 MB	192.00 MB	200.00 MB	208.00 MB	224.00 MB	232.00 MB	248.00 MB	296.00 MB	320.00 MB	376.00 MB
gloo_speed(GB/s)	0.90	1.53	1.90	2.16	2.18	2.45	2.60	2.60	2.69	2.66	2.74	3.00	2.82	2.84	3.22	2.49	2.36	2.67	2.66	2.60	2.28	2.52	2.50	2.10	1.34	3.11	2.68	1.60	2.57	2.22	0.86	1.84

migration_size	8.00 MB	16.00 MB	24.00 MB	32.00 MB	40.00 MB	48.00 MB	56.00 MB	64.00 MB	72.00 MB	80.00 MB	88.00 MB	96.00 MB	104.00 MB	112.00 MB	120.00 MB	128.00 MB	136.00 MB	144.00 MB	152.00 MB	160.00 MB	168.00 MB	176.00 MB	184.00 MB	192.00 MB	200.00 MB	208.00 MB	216.00 MB	232.00 MB	240.00 MB	256.00 MB	296.00 MB	320.00 MB
nccl_speed(GB/s)	0.20	0.43	0.68	0.84	1.00	1.19	1.52	1.47	1.84	1.95	1.69	1.95	1.90	2.29	2.39	2.00	2.84	2.23	2.65	2.73	2.94	3.40	3.98	2.50	3.51	3.44	4.34	3.02	4.85	3.10	2.64	2.74