Update ROCm build pre-built packages instructions

[Looong01]

Motivation

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

Modification

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

Checklist

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.