Add more info to the development index file

Add more info to the development index file

Credit where credit is due; I inspired myself by looking at the yunohost docs
* https://yunohost.org/en/dev
* https://yunohost.org/en/packaging_apps_start

I try to be inviting to new developers and guide them in their first steps into Akkoma development.
I try to keep the page itself as short as possible and link to relevant places.
That way people can quickly skim over parts that they don't need, while people who do need more can simply follow the links.