[mantine.dev] Add OpenCollective mention to getting started page

apps/mantine.dev/src/pages/getting-started.mdx

@@ -194,6 +194,8 @@ The most important documentation pages are:
 All contributions to the projects are welcome and appreciated.
 There are many ways to support the project:
 
+- Contribute financially by [sponsoring the project on OpenCollective](https://opencollective.com/mantinedev).
+  Your sponsorship will help us to maintain the project and develop new features.
 - Share your feedback in [GitHub Discussions](https://github.com/mantinedev/mantine/discussions/categories/feedback) –
   we are always happy to hear your thoughts on how to improve Mantine. Most of the new features and components
   are based on the feedback received from the community.