chore: Add REUSE Compliance

[KhronosWebservices]

• add reuse GitHub workflow action
• setup reuse compliance

[SaschaWillems]

@KhronosWebservices : We just merged a PR that adds a .webp image and are now getting the following CI failure:

The following files have no copyright and licensing information:
* chapters/images/slang-write-once-run-anywhere-graphic.webp

What would be required to get this fixed? I obviously can't add a copyright header to a binary image file.

[allcreatures]

3 options:

1. If all chapters files will be the same copyright/license, changes:

 "chapters/**.md",
    "chapters/**.png",
    "chapters/**.svg",

to

 "chapters/**",

2. If the webp has a different copyright license, adding a slang-write-once-run-anywhere-graphic.webp.license file with the specific copyright/license is one way, or
3. If the webp has a different copyright license, adding a slang-write-once-run-anywhere-graphic.webp.license to REUSE.toml with the correct copyright license is the other way.

If the entire repository is CC-BY-4.0 with Copyright The Khronos Group, Inc. with a few exceptions, then adding a catch all annotation at the top of the REUSE.toml is the way to go:

[[annotations]]
path = [
    "**",
]
Precedence: "closest"
SPDX-FileCopyrightText = "2026 The Khronos Group, Inc."
SPDX-License-Identifier = "CC-BY-4.0"

The closest precedence will let files with their own copyright/license take precedence, and REUSE.toml annotations cascade, so annotations lower down take precedence over annotations higher up.