Update ROCm build pre-built packages instructions

Motivation

Add the external repo link of mmcv-rocm-build's pre-built packages.

Add the links to README.md and README_zh-CN.md.

Before PR:

[*] I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
[*] Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
[*] Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
[*] New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
[*] The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

[*] If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
[*] CLA has been signed and all committers have signed the CLA in this PR.