manual on official site outdated

**Describe the bug**
The link on doc/README.md is going to https://facebook.github.io/zstd/zstd_manual.html which has the 1.5.1 manual, while the one that is linked from the website itself points to the 1.5.2 manual at https://raw.githack.com/facebook/zstd/release/doc/zstd_manual.html.

**Expected behavior**
Both links pointing to the current version, ideally "automatic".