Add notes for installing "extended" Sass/SCSS version

[anthonyfok]

This PR adds instruction for installing "extended" Sass/SCSS version, especially the new "extended" track of Hugo Snap, and made some other revisions along the way.

It is not comprehensive, e.g. the Homebrew version is now built with "extended" but I haven't edited those instructions yet. I was also thinking of splitting installing.md into two separate pages "installing pre-built binary" and "building from source". Too much work, maybe next time. ;-)

@bep, my apologies for not having a discussion with you since we last discussed this two weeks ago at gohugoio/hugo#4908. The Snap gurus at https://forum.snapcraft.io/t/request-for-new-extended-track-for-hugo-snap/6297 has decided that the use of snap "track" is the best way to go after reading our prior discussions, and have set up one for us after a one-week voting period, and I have been testing it for a few days and everything is working flawlessly.

But yes, I thought I'd better let you know what is going on before publicizing snap install hugo --channel=extended, hence this PR. :-)

For your information, these are being built from Launchpad again automatically:

• https://launchpad.net/~gohugoio/+snap/hugo-extended
  • built from "extended-snap-stable" branch
  • uploads to "extended/stable" channel
• https://launchpad.net/~gohugoio/+snap/hugo-extended-dev
  • built from "extended-snap-master" branch
  • uploads to "extended/edge" channel

$ snap info hugo
name:    hugo
summary: A Fast and Flexible Static Site Generator built with love by bep, spf13 and friends
  in Go.
publisher: hugo-authors
contact:   https://discourse.gohugo.io/
license:   unknown
description: |
  Hugo is a static HTML and CSS website generator written in Go. It is
  optimized for speed, easy use and configurability. Hugo takes a directory
  with content and templates and renders them into a full HTML website.
commands:
  - hugo
snap-id:      t6YzDBM3com27avINGL5OD6ZKKG654ez
tracking:     extended/stable
refresh-date: today at 10:17 MDT
channels:                                  
  stable:             0.45     (2338) 33MB -
  candidate:          ↑                    
  beta:               ↑                    
  edge:               0.46-DEV (2335) 33MB -
  extended/stable:    0.45     (2350) 33MB -
  extended/candidate: ↑                    
  extended/beta:      ↑                    
  extended/edge:      0.46-DEV (2351) 33MB -
installed:            0.45     (2350) 33MB -

(The "license: unknown" line is a glitch with the snap CLI. The Snap Store web interface correctly shows Apache-2.0 for our license.)

I currently run git rebase for these extended-snap-* branches until I start to learn how to do it automatically via git hooks.

Many thanks for reviewing, and thank you for Hugo 0.45!