chore: update documentation and notice year to 2026 (flashinfer-ai#2285)

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

## 📌 Description

2025 -> 2026.

## 🔍 Related Issues

<!-- Link any related issues here -->

## 🚀 Pull Request Checklist

Thank you for contributing to FlashInfer! Before we review your pull
request, please make sure the following items are complete.

### ✅ Pre-commit Checks

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

> If you are unsure about how to set up `pre-commit`, see [the
pre-commit documentation](https://pre-commit.com/).

## 🧪 Tests

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

## Reviewer Notes

<!-- Optional: anything you'd like reviewers to focus on, concerns, etc.
-->


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Chores**
* Updated copyright year ranges to 2026 in project headers and
documentation.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: yzh119

NOTICE

@@ -1,6 +1,6 @@
 FlashInfer
-Copyright 2025 NVIDIA
-Copyright 2023-2025 FlashInfer community (https://flashinfer.ai/)
+Copyright 2025-2026 NVIDIA
+Copyright 2023-2026 FlashInfer community (https://flashinfer.ai/)
 
 -------------------------------------------------------------------------------------------------
 Some of the code in this project are adapted from other open-source projects with different

docs/conf.py

@@ -24,7 +24,7 @@
 
 project = "FlashInfer"
 author = "FlashInfer Contributors"
-copyright = f"2023-2025, {author}"
+copyright = f"2023-2026, {author}"
 
 version = flashinfer.__version__
 release = flashinfer.__version__