docs: update Alpha 3 release notes and fix Overleaf description

Add top/bottom split view and bidirectional format export (LaTeX↔Typst,
LaTeX→DOCX/Markdown/HTML) to the Alpha 3 feature list. Fix the security
section which incorrectly referred to Overleaf as a LaTeX/Typst editor —
Overleaf only supports LaTeX.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -147,6 +147,11 @@ Refinements to the Typst editor and the format badge system:
 - **RevealJS thumbnails** — first-slide PNG preview rendered in project cards.
 - **Upload reliability** — streaming response fix so large file uploads on slow
   connections no longer time out at the proxy layer.
+- **Top/bottom split view** — new editor layout that stacks the source editor
+  above the PDF preview vertically, in addition to the existing side-by-side mode.
+- **Bidirectional format export** — LaTeX projects can be exported to Typst,
+  DOCX, Markdown or HTML via pandoc; Typst projects can be exported to LaTeX.
+  Available from the File menu in the editor.
 
 ---
 
@@ -163,7 +168,7 @@ Refinements to the Typst editor and the format badge system:
 > There is **no per-project sandbox or resource isolation** beyond what the
 > operating system provides. Exposing Verso to the public internet with open
 > registration is not recommended. If you need to host a collaborative
-> LaTeX/Typst editor for untrusted users or at scale, look at
+> LaTeX editor for untrusted users or at scale, look at
 > [Overleaf's non-Community offerings](https://www.overleaf.com/for/enterprises),
 > which include proper sandboxing and enterprise access controls.