Add a "what's new" page to the devguide

During the last docs-community meeting, we discussed the fact that we are currently using several different tools (git, GitHub, Discourse, Discord, etc.) and each tool has a learning curve. Combined with the fact that the tools change and evolve over time, it's difficult for the devs to use them proficiently and effectively.

Documenting how to use them in the devguide is useful, but devs rarely re-read the devguide to discover these new sections. I therefore propose to create a "What's new" page to list significant additions and changes to the devguide, to make these more discoverable.

I think it will also be useful to post these entries with a regular cadence (e.g. monthly) on Discourse, so that people don't have to actively check the "What's new" page and remember when was the last time they checked.