[skip changelog] Sync install docs from "template" (#1544)

* Add an introduction to install docs

This document provides a list of installation options. The user may pick any one option that is most convenient for them.
However, a reader might get the impression that it is a list of instructions that must be followed from top to bottom.

An introduction makes the situation immediately clear to the reader.

* Clarify that sh is not required in template install docs

A significant portion of Arduino users use Windows. The first option listed in the installation instructions is for an
installation shell script, which can be used on Windows, but might not be convenient for some Windows users.

This could be discouraging or frustrating to potential new users, but not necessarily so because the script is an
optional convenience and directly downloading and installing the tool is also an option, as documented in the
instructions for the second listed option.

A little refinement of the wording of the script's `sh` requirement makes the situation clear to those users.

* Add a reference link re: PATH to install docs

A reference link provides easy access to the relevant information for readers who are not familiar with this concept
while not harming readability for everyone else by increasing verbosity.

* Align heading levels in install docs

This document is intended to provide a list of installation options:

- brew
- script
- download
- build

but the heading levels were misaligned, which resulted in this structure:

-brew
- script
  - download
  - build

Author: per1234

docs/installation.md

@@ -1,5 +1,7 @@
 <!-- Source: https://github.com/arduino/tooling-project-assets/blob/main/other/installation-script/installation.md -->
 
+Several options are available for installation of Arduino CLI. Instructions for each are provided below:
+
 ## Install via Homebrew (macOS/Linux)
 
 The Arduino CLI is available as a Homebrew formula since version `0.5.0`:
@@ -15,8 +17,9 @@ brew install arduino-cli
 
 ## Use the install script
 
-The script requires `sh`, which is always available on Linux and macOS. `sh` is not available by default on Windows. The
-script can be run on Windows by installing [Git for Windows](https://gitforwindows.org/), then running it from Git Bash.
+The script requires `sh`, which is always available on Linux and macOS. `sh` is not available by default on Windows,
+though it is available as part of [Git for Windows](https://gitforwindows.org/) (Git Bash). If you don't have `sh`
+available, use the ["Download" installation option](#download).
 
 This script will install the latest version of Arduino CLI to `$PWD/bin`:
 
@@ -31,7 +34,8 @@ curl -fsSL https://raw.githubusercontent.com/arduino/arduino-cli/master/install.
 ```
 
 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.
+[`PATH`](https://en.wikipedia.org/wiki/PATH%5F%28variable%29) or add the Arduino CLI installation path to your `PATH`
+environment variable.
 
 If you want to download a specific Arduino CLI version, for example `0.9.0` or `nightly-latest`, pass the version number
 as a parameter like this:
@@ -44,14 +48,15 @@ Arduino CLI checks for new releases every 24 hours. If you don't like this behav
 [`updater.enable_notification` config](configuration.md#configuration-keys) or the
 [env var `ARDUINO_UPDATER_ENABLE_NOTIFICATION`](configuration.md#environment-variables) to `false`.
 
-### Download
+## 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.
+in your [`PATH`](https://en.wikipedia.org/wiki/PATH%5F%28variable%29) or add the Arduino CLI installation path to your
+`PATH` environment variable.
 
-#### Latest release
+### Latest release
 
 | Platform  |                      |                      |
 | --------- | -------------------- | -------------------- |
@@ -72,11 +77,11 @@ in your `PATH` or add the Arduino CLI installation path to your `PATH` environme
 > `http://downloads.arduino.cc/arduino-cli/arduino-cli-latest-<platform>.tar.bz2` won’t be further updated. That URL
 > will provide the version `0.3.7-alpha.preview`, regardless of further releases.
 
-#### Previous versions
+### Previous versions
 
 These are available from the "Assets" sections on the [releases page](https://github.com/arduino/arduino-cli/releases).
 
-#### Nightly builds
+### Nightly builds
 
 These builds are generated every day at 01:00 GMT from the `master` branch and should be considered unstable. In order
 to get the latest nightly build available for the supported platform, use the following links:
@@ -102,7 +107,7 @@ to get the latest nightly build available for the supported platform, use the fo
 Checksums for the nightly builds are available at
 `https://downloads.arduino.cc/arduino-cli/nightly/nightly-<DATE>-checksums.txt`
 
-### Build from source
+## Build from source
 
 If you're familiar with Golang or if you want to contribute to the project, you will probably build Arduino CLI locally
 with your Go toolchain. See the ["How to contribute"](CONTRIBUTING.md#building-the-source-code) page for instructions.