Migrate from Discourse to Github Discussions & Projects

> [!IMPORTANT]
> This is a [roadmap](https://doomemacs.org/roadmap) issue and is used solely to announce and document progress (to make it easier for users to follow and maintainers to track). Please do not use it for discussion; use [our discussion board](https://doomemacs.org/discuss) instead.

> [!NOTE]
> This was [announced and started on June 22, 2025](https://github.com/orgs/doomemacs/discussions/1).

## Tasks

- [X] Open a central Github Discussions board ([done](https://doomemacs.org/discuss)).
- [X] Create an announcement post about the choice.
- [X] Put Discourse in read-only mode.
- [X] Update all former links to Discourse.
- [X] Transfer our [Do-Not-PR list](https://discourse.doomemacs.org/t/do-not-pr/2406) ([done](https://doomemacs.org/donotpr))
- [ ] Transfer our [Development roadmap](https://discourse.doomemacs.org/t/development-roadmap/42)
- [ ] #3
- [ ] Update docs.doomemacs.org ([repo](https://github.com/doomemacs/docs))
- [ ] Deploy wiki.doomemacs.org ([repo](https://github.com/doomemacs/wiki))
- [ ] Transfer [old guides and tutorials](https://discourse.doomemacs.org/c/guides/5) (to either our official documentation, community wiki, or a pinned discussion).
- [ ] Integrate [giscus](https://github.com/giscus/giscus) into wiki.doomemacs.org (comments).
- [ ] Integrate [lunrjs](https://lunrjs.com/guides/getting_started.html) into (docs|wiki).doomemacs.org (text search).