fix(doc): switch from google groups to discord

## Initial release of a new template (advanced)

1. Start posting a message with topic `New template: bla bla` on the [to be continuous Google Group](https://groups.google.com/g/tbc-dev),

1. Start posting a message with topic `New template: bla bla` on the [to be continuous Discord channel](https://discord.gg/SetvpZ9XZ6),

and present the scope and goal of your template.

2. If agreed by the core team, a new project will be created from [template skeleton](https://gitlab.com/to-be-continuous/tools/template-skeleton)

   with private visibility in the [to be continuous GitLab group](https://gitlab.com/to-be-continuous). You'll be granted `developer` access to it.