docs(repo): remove obsolete CodeSandbox contribution workflow (#11908)

* docs: remove obsolete CodeSandbox contribution workflow

https://codesandbox.io/p/github/excalidraw/excalidraw (forking the
whole repo as an editable sandbox to submit PRs from) no longer works.
It was documented in two places — the Contributing guide and the
Development guide — as an alternative to the manual clone/fork setup.

This doesn't touch the other CodeSandbox links in the docs (README,
installation/integration/development guides for the @excalidraw/excalidraw
package), which point at the separate with-script-in-browser usage example
and are unrelated to this contribution workflow.

* chore: trigger CI

Claude Opus 5.5 (low)

---------

Co-authored-by: dwelle <5153846+dwelle@users.noreply.github.com>

dev-docs/docs/introduction/contributing.mdx

@@ -9,8 +9,6 @@ In case you want to pick up something from the roadmap, comment on that issue an
 
 ## Setup
 
-### Option 1 - Manual
-
 1. Fork and clone the repo
 1. Run `yarn` to install dependencies
 1. Create a branch for your PR with `git checkout -b your-branch-name`
@@ -23,15 +21,6 @@ In case you want to pick up something from the roadmap, comment on that issue an
 > git branch --set-upstream-to=upstream/master master
 > ```
 
-### Option 2 - CodeSandbox
-
-1. Go to https://codesandbox.io/p/github/excalidraw/excalidraw
-1. Connect your GitHub account
-1. Go to Git tab on left side
-1. Tap on `Fork Sandbox`
-1. Write your code
-1. Commit and PR automatically
-
 ## Pull Request Guidelines
 
 Don't worry if you get any of the below wrong, or if you don't know how. We'll gladly help out.

dev-docs/docs/introduction/development.mdx

@@ -1,11 +1,5 @@
 # Development
 
-## Code Sandbox
-
-- Go to https://codesandbox.io/p/github/excalidraw/excalidraw
-  - You may need to sign in with GitHub and reload the page
-- You can start coding instantly, and even send PRs from there!
-
 ## Local Installation
 
 These instructions will get you a copy of the project up and running on your local machine for development and testing purposes.