Documentation links in README are unreachable

**Description:**
I found that the documentation link in the README is currently unreachable:

```
https://magicstack.github.io/async_gaussdb/current/installation.html
```

This URL returns a 404 / cannot be accessed.

**Expected Behavior:**
The link should point to a valid documentation page for installation and usage.

**Actual Behavior:**
The link is broken and does not load.
