Add NOTICE with copyrights (flashinfer-ai#1432)

<!-- .github/pull_request_template.md -->

## 📌 Description

Adds NOTICE file to specify copyrights for both NVIDIA and Flashinfer
community.

Follows: https://infra.apache.org/licensing-howto.html#example-notice

## 🔍 Related Issues

None

## 🚀 Pull Request Checklist

### ✅ Pre-commit Checks

- [ ✅ ] I have installed `pre-commit` by running `pip install
pre-commit` (or used your preferred method).
- [ ✅ ] I have installed the hooks with `pre-commit install`.
- [ ✅ ] I have run the hooks manually with `pre-commit run --all-files`
and fixed any reported issues.

## 🧪 Tests

- [ N/A ] Tests have been added or updated as needed.
- [ N/A ] All tests are passing (`unittest`, etc.).

## Reviewer Notes

Check dates and naming of "Flashinfer community"

Author: sricketts

NOTICE

@@ -0,0 +1,8 @@
+FlashInfer
+Copyright 2025 NVIDIA
+Copyright 2023-2025 FlashInfer community (https://flashinfer.ai/)
+
+-------------------------------------------------------------------------------------------------
+Some of the code in this project are adapted from other open-source projects with different
+licenses. This product also bundles some third-party components under other open source licenses.
+See licenses/ for text of these licenses.