Update guideline links in sustainability_considerations.md (#12762)

One of your links was busted, so I figured I'd suggest some updates.

CONTRIBUTORS.md

@@ -866,6 +866,7 @@
 * Unyime Emmanuel Udoh
 * Joren Hammudoglu
 * Rishabh Sharma
+* Mike Gifford
 
 ## Translators
 

docs/advanced_topics/sustainability_considerations.md

@@ -15,13 +15,12 @@ Those are the same standards used to assess the [sustainability of Wagtail](http
 
 Here are the guidelines we would recommend applying to Wagtail websites:
 
--   [Sustainable Web Design Strategies](https://sustainablewebdesign.org/strategies/)
+-   [Sustainable Web Design W3C Interest Group](https://www.w3.org/groups/ig/sustainableweb/) working draft of the [Web Sustainability Guidelines](https://w3c.github.io/sustainableweb-wsg/)
+-   [Sustainable Web Design](https://sustainablewebdesign.org/)
 -   [GR491](https://gr491.isit-europe.org/en/)
 -   [Green Design Principles by Microsoft (PDF)](https://wxcteam.microsoft.com/download/Microsoft-Green-Design-Principles.pdf)
 -   [Green Software Foundation Patterns](https://patterns.greensoftware.foundation/catalog/web/)
 
-The [Sustainable Web Design W3C Community Group](https://www.w3.org/community/sustyweb/) is also working on _Web Environmental Sustainability Guidelines_, which will be highly relevant once published.
-
 ## Quantifying emissions
 
 To quantify the emissions of a Wagtail website, we recommend three different approaches: