Add alert blocks in markdown (#29121)
- Follows https://github.com/go-gitea/gitea/pull/21711
- Closes https://github.com/go-gitea/gitea/issues/28316
Implement GitHub's alert blocks markdown feature
Docs:
-
https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts
- https://github.com/orgs/community/discussions/16925
### Before
### After
## ⚠️ BREAKING ⚠️
The old syntax no longer works
How to migrate:
If you used
```md
> **Note** My note
```
Switch to
```md
> [!NOTE]
> My note
```
---------
Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>
