docs: add welcome plugin docs
| stage | | TODO |
| trigger | `triggers` | TODO |
| updateconfig | `config_updater` | TODO |
-| welcome | `welcome` | TODO |
+| welcome | `welcome` | [docs](./plugins/welcome.md) |
| wip | | [docs](./plugins/wip.md) |
| yuks | | [docs](./plugins/yuks.md) |
