Upgrade CANN version to 8.1.rc1 (#747)

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

Make CANN version bump separately from
https://github.com/vllm-project/vllm-ascend/pull/708

- Upgrade CANN version to 8.1.rc1
- Add prefix to speed up download
`m.daocloud.io/quay.io/ascend/cann:8.1.rc1-910b-ubuntu22.04-py3.10`
- Address tail sapce for Dockerfile.openEuler
- Add note for `/workspace` and `/vllm-workspace` as followup of
https://github.com/vllm-project/vllm-ascend/pull/741

### Does this PR introduce _any_ user-facing change?
no

### How was this patch tested?

CI passed

Co-authored-by: MengqingCao <cmq0113@163.com>

Signed-off-by: Yikun Jiang <yikunkero@gmail.com>
Co-authored-by: MengqingCao <cmq0113@163.com>

docs/source/conf.py

@@ -73,7 +73,7 @@ myst_substitutions = {
     'pip_vllm_ascend_version': "0.8.4rc2",
     'pip_vllm_version': "0.8.4",
     # CANN image tag
-    'cann_image_tag': "8.0.0-910b-ubuntu22.04-py3.10",
+    'cann_image_tag': "8.1.rc1-910b-ubuntu22.04-py3.10",
 }
 
 # Add any paths that contain templates here, relative to this directory.