Support alert blocks for org-mode rendering
## Problem
`#+BEGIN_QUOTE` blocks in `.org` files with `[!note]` / `[!tip]` / `[!important]` / `[!caution]` / `[!warning]` syntax are rendered as plain blockquotes. In Markdown (`.md`), the equivalent `> [!note]` is [rendered as styled alert blocks](https://docs.gitlab.com/user/markdown/#alerts).
GitHub already supports alert blocks for org-mode, so similar support in GitLab would be very helpful.
## Expected vs Actual
**Expected rendering (GitHub)**:
{width=384 height=87}
**Actual rendering (GitLab)**:
{width=431 height=47}
**Source**:
```org
#+BEGIN_QUOTE
[!warning]
GitLab alert syntax is not rendered in org-mode.
#+END_QUOTE
```
## Possible approach
- Add a new Banzai filter (e.g. `MarkupAlertFilter`) to [MarkupPipeline](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/banzai/pipeline/markup_pipeline.rb)
Related to #251204
issue