Update roadmap links for DataFusion Q1 2026 (#18495)

## Which issue does this PR close?
N/A
## Rationale for this change

I started a new roadmap discussion, so let's also link it in the docs

## What changes are included in this PR?

Add a link to
- #18494

## Are these changes tested?
by C I

## Are there any user-facing changes?

new link in docs

Author: alamb

docs/source/contributor-guide/roadmap.md

@@ -55,8 +55,9 @@ discussion.
 For more information:
 
 1. [Search for issues labeled `roadmap`](https://github.com/apache/datafusion/issues?q=is%3Aissue%20%20%20roadmap)
-2. [DataFusion Road Map: Q3-Q4 2025](https://github.com/apache/datafusion/issues/15878)
-3. [2024 Q4 / 2025 Q1 Roadmap](https://github.com/apache/datafusion/issues/13274)
+2. [DataFusion Road Map: Q1 2026](https://github.com/apache/datafusion/issues/18494)
+3. [DataFusion Road Map: Q3-Q4 2025](https://github.com/apache/datafusion/issues/15878)
+4. [2024 Q4 / 2025 Q1 Roadmap](https://github.com/apache/datafusion/issues/13274)
 
 ## Improvement Proposals