Update install instructions for alpine without wheels

Updated to match the format of the dependencies with wheels.

Also added `libwebp-dev` to the build dependencies. Otherwise building would work but running

```
weasyprint https://weasyprint.org/ /tmp/test.pdf
```

would fail to include the webp images with error code
```
/usr/lib/python3.10/site-packages/PIL/Image.py:3496: UserWarning: image file could not be identified because WEBP support not installed
```

The prebuilt `Pillow` binary in the wheel includes `webp` support already.
```

docs/first_steps.rst

@@ -85,7 +85,8 @@ the following packages::
 To install WeasyPrint inside a virtualenv without using wheels, you need the
 following packages::
 
-  apk add py3-pip gcc musl-dev python3-dev pango zlib-dev jpeg-dev openjpeg-dev g++ libffi-dev harfbuzz-subset
+  apk add py3-pip so:libgobject-2.0.so.0 so:libpango-1.0.so.0 so:libharfbuzz.so.0 so:libharfbuzz-subset.so.0 so:libfontconfig.so.1 so:libpangoft2-1.0.so.0
+  apk add gcc musl-dev python3-dev zlib-dev jpeg-dev openjpeg-dev libwebp-dev g++ libffi-dev
 
 
 Archlinux