Add explainer

[hidde]

For the TAG review, we'll need an explainer, a document that explains in clear language what we do, and covers what the problem is and how we solve it (see explainer explainer).

I've based this on our summary and our abstract and introduction, focusing on the specific questions asked in the explainer explainer.

[TzviyaSiegman]

internationalization (US spelling)

[hidde]

Suggested change

	Where applicable, accessibility, internationalisation, privacy, and security have been considered throughout development of the Web Sustainability Guidelines. [A list of guidelines to these areas](https://w3c.github.io/sustainableweb-wsg/#considerations) is available.
	Where applicable, accessibility, internationalization, privacy, and security have been considered throughout development of the Web Sustainability Guidelines. [A list of guidelines to these areas](https://w3c.github.io/sustainableweb-wsg/#considerations) is available.