bpo-36329: Remove 'make -C Doc serve' in favour of 'make -C Doc htmlview' (GH-32354)

Also updated `make -C htmlview` so it used a full path with `file://`, because the original didn't open the page (macOS).

For example:
```sh
cd Doc

# Doesn't open anything:
python3 -c "import webbrowser; webbrowser.open('build/html/index.html')"

# Opens the docs page e.g. file:///Users/hugo/github/cpython/Doc/build/html/index.html :
python3 -c "import os, webbrowser; webbrowser.open('file://' + os.path.realpath('build/html/index.html'))"
```

https://bugs.python.org/issue36329