docs: no contributions/donations, add security model + Collabst + ecosystem support

- Remove Contributing section (not accepting PRs/issues)
- Add Security model section: Verso is for trusted environments only;
  point untrusted-user use cases at Overleaf non-Community offerings
- Mention Collabst as a promising open-source Typst-only alternative
  in the Verso vs Typst.app comparison
- Add Supporting the ecosystem section redirecting to Overleaf (Typst +
  RevealJS work) and the Typst project instead of Verso donations

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

README.md

@@ -66,6 +66,11 @@ Verso differs in that:
   gracefully for long documents.
 - It ships with a full **project history** and version-restore workflow.
 
+If you only need Typst and want a lighter, Typst-focused alternative, have a look
+at **[Collabst](https://github.com/herluf-ba/collabst)** — an open-source,
+self-hosted collaborative Typst editor that is independent of the Overleaf
+codebase and shows a lot of promise.
+
 ---
 
 ## Features
@@ -145,6 +150,24 @@ Refinements to the Typst editor and the format badge system:
 
 ---
 
+## Security model — trusted environments only
+
+Verso is designed for **closed groups of trusted users** (a lab, a class, a small
+team). All three compilers can execute arbitrary code on the server:
+
+- LaTeX with shell-escape enabled can run system commands.
+- Quarto Python cells execute Python code directly.
+- Typst's scripting layer is sandboxed by design, but runs server-side.
+
+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
+[Overleaf's non-Community offerings](https://www.overleaf.com/for/enterprises),
+which include proper sandboxing and enterprise access controls.
+
+---
+
 ## Quick start
 
 ### With Docker
@@ -235,11 +258,20 @@ Verso is not affiliated with Overleaf Ltd.
 
 ---
 
-## Contributing
+## Supporting the ecosystem
 
-Open an issue or pull request on the
-[Verso repository](https://git.alocoq.fr/alois/verso). The upstream Overleaf
-contribution guidelines are in [CONTRIBUTING.md](CONTRIBUTING.md).
+Verso is not accepting contributions or donations at this time. If you find it
+useful and want to support the broader ecosystem it builds on:
+
+- **Support Overleaf** — Overleaf is actively working on
+  [Typst support](https://www.overleaf.com/blog/overleaf-and-typst) and
+  RevealJS presentation features. The best way to support their work is to use
+  or subscribe to [Overleaf](https://www.overleaf.com) and encourage your
+  institution to do the same.
+- **Support Typst** — [Typst GmbH](https://typst.app) is the company behind the
+  Typst compiler. Using Typst.app or sponsoring the
+  [Typst project on GitHub](https://github.com/typst/typst) helps sustain the
+  language itself.
 
 ---