bpo-36329: Remove 'make -C Doc serve' in favour of 'make -C Doc htmlview'

[hugovk]

Matching Devguide PR: python/devguide#826.

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

For example:

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

[encukou]

Could you make serve echo The serve target was removed, use htmlview instead (see [bpo-36329](https://bugs.python.org/issue36329)), to help anyone still using it?
Also, could you keep the example in wsgiref docs? It has a purpose there. (Maybe it could be simplified to focus on serving, better commented, and moved to Doc/, of course. But it shouldn't be removed.)

[hugovk]

Updated! How does this look?

[encukou]

Looks great, thank you!

Sorry for the delay, this fell off my list :(

[hugovk]

Thanks for the review/merge! Any chance of checking the devguide one? python/devguide#826