doc: Add build instruction on AlpineLinux

[seia-soto]

• https://discuss.linuxcontainers.org/t/add-documentation-of-incus-compile-instruction-on-alpinelinux/18391
• I'm not a member of AlpineLinux
• I currently do not have enough knowledge on AlpineLinux packages and the ability to manage packages

This PR adds Incus build instruction on AlpineLinux.

Motivation

AlpineLinux is lightweight Linux distro suitable for containers and virtualization even as host operating system.
Add build and running instruction of Incus on AlpineLinux, so Incus can be used by wider users.

See the forum reference link above for my personal use-case of AlpineLinux and Incus.

Possible considerations on PR

Heading prefix of instruction on other Linux distros

I used Other distros: [distro name here] in the commit but I'd like to accept suggestion for better prefix. The final ToC will look like:

Install Incus from source
From source: Build the latest version
From source: Build a release
Start the build
From source: Install
Machine setup
+ Other distros: AlpineLinux
+ > make: gettext.cgo2.c:(.text+0x2b9): undefined reference to `libintl_gettext’

Emphasis as header

The commit includes 4th heading to describe why specific commands is used to resolve build time issues. If you think it's going to mess up the ToC, consider to allow the rule MD036 or replace with list.

% make doc-lint | grep -i installing.md
.tmp/doc/installing.md:273: MD036 Emphasis used instead of a header

Currently lint result on installing.md has no problem.

• Current

#### `make`: gettext.cgo2.c:(.text+0x2b9): undefined reference to `libintl_gettext'

• Allowing the rule MD036

**`make`: gettext.cgo2.c:(.text+0x2b9): undefined reference to `libintl_gettext'**

• Using list

- `make`: gettext.cgo2.c:(.text+0x2b9): undefined reference to `libintl_gettext'

Future considerations

• Adding Incus to AlpineLinux/aports for high quality package distribution. (AlpineLinux communication channels: https://wiki.alpinelinux.org/wiki/Alpine_Linux:IRC)
• Adding tabs per Linux distros by @ stgraber on forum-18391. (https://discuss.linuxcontainers.org/t/add-documentation-of-incus-compile-instruction-on-alpinelinux/18391/2?u=hojeong-go)

[seia-soto]

Added sign off message in the commit as CONTRIBUTING.md describes.

[stgraber]

Thanks!

I'll send a follow-up PR which does some re-organization on that page to make it less Ubuntu/Debian specific overall and make it easier to get the instructions for whatever distribution you're running.

[seia-soto]

Thanks for your effort!