[skip changelog] Clarify installation instructions for Windows (#706)

* [skip changelog] Document the install script's compatibility with Windows

It's already quite clear that the Homebrew installation method is not available for Windows, so the first option for Windows users is the install script, which is also not well supported on Windows.

* [skip changelog] Make the PATH instructions more prominent in the installation docs

Although the installation script does recommend adding Arduino CLI to PATH, I though it worth providing an explanation of why in installation instructions.

I combined the various download sources into sub-headings of a single "Download" installation option section so that the PATH instructions could be provided prominently only once for that installation option.

Author: per1234

docs/installation.md

@@ -10,27 +10,37 @@ brew install arduino-cli
 
 ### Use the install script
 
-The easiest way to get the latest version of the Arduino CLI on any
-supported platform is using the `install.sh` script:
+The script requires `sh`. This is always available on Linux and macOS. `sh` is
+not available by default on Windows. The script may be run on Windows by
+installing [Git for Windows], then running it from Git Bash.
+
+This script will install the latest version of Arduino CLI to `$PWD/bin`:
 
 ```sh
 curl -fsSL https://raw.githubusercontent.com/arduino/arduino-cli/master/install.sh | sh
 ```
 
-The script will install `arduino-cli` at `$PWD/bin` but if you want to target a
-different directory, for example `~/local/bin`, set the `BINDIR` environment
-variable like this:
+If you want to target a different directory, for example `~/local/bin`, set the
+`BINDIR` environment variable like this:
 
 ```sh
 curl -fsSL https://raw.githubusercontent.com/arduino/arduino-cli/master/install.sh | BINDIR=~/local/bin sh
 ```
 
-### Download the latest packages
+If you would like to use the `arduino-cli` command from any location, install
+Arduino CLI to a directory already in your `PATH` or add the Arduino CLI
+installation path to your `PATH` environment variable.
+
+### Download
+
+Pre-built binaries for all the supported platforms are available for download
+from the links below.
+
+If you would like to use the `arduino-cli` command from any location, extract
+the downloaded file to a directory already in your `PATH` or add the Arduino CLI
+installation path to your `PATH` environment variable.
 
-You can download the latest version of the pre-built binaries for the supported
-platforms from the [release page](https://github.com/arduino/arduino-cli/releases)
-or following the links in the following table. Once downloaded, extract the
-binary `arduino-cli` into a directory that is in your `PATH`.
+#### Latest packages
 
 Platform  |                    |                    |
 --------- | ------------------ | ------------------ |
@@ -52,7 +62,11 @@ Mac OSX   |                    | [Mac OSX]          |
   won’t be further updated. That URL will provide the version
   `0.3.7-alpha.preview`, regardless of further releases.
 
-### Download a nightly build
+#### Previous versions
+
+These are available from the [releases page](https://github.com/arduino/arduino-cli/releases)
+
+#### Nightly builds
 
 These builds are generated everyday at 01:00 GMT from the `master` branch and
 should be considered unstable. In order to get the latest nightly build
@@ -81,9 +95,6 @@ Mac OSX   |                            | [Mac OSX]                  |
 Checksums for the nightly builds are available at
 `https://downloads.arduino.cc/arduino-cli/nightly/nightly-<DATE>-checksums.txt`
 
-Once downloaded, extract the executable `arduino-cli` into a directory
-which is in your ``PATH``.
-
 ### Build from source
 
 If you’re familiar with Golang or if you want to contribute to the
@@ -100,3 +111,5 @@ docker run -v $PWD:/arduino-cli -w /arduino-cli -e PACKAGE_NAME_PREFIX='snapshot
 
 Once the build is over, you will find a `./dist/` folder containing the packages
 built out of the current source tree.
+
+[Git for Windows]: https://gitforwindows.org/