Migrate draft to new mdx format

Author: AdamGS

src/content/blog/september-2025.md renamed to src/content/blog/september-2025.mdx

@@ -14,7 +14,7 @@ Development keeps moving quickly, with many new features and improvements of the
 
 ## Core
 
-1. We've published an [RFC](<(https://github.com/vortex-data/vortex/pull/4726)>) presenting our planned changes to how arrays and compute interact with each other, introducing what we call Operators.
+1. We've published an [RFC](https://github.com/vortex-data/vortex/pull/4726) presenting our planned changes to how arrays and compute interact with each other, introducing what we call Operators.
 1. We merged a few big changes to the IO APIs ([#4557](https://github.com/vortex-data/vortex/pull/4608), [#4608](https://github.com/vortex-data/vortex/pull/4608)), which introduce new push-based write API (in addition to the existing stream-based one), and writes now return both the file's footer (which can now be cached, reused and even de/serialized [#4598](https://github.com/vortex-data/vortex/pull/4608)), and the file's total compressed size.
 1. We've added a new encoding - [fastlanes](https://github.com/spiraldb/fastlanes)-based RLE [#4588](https://github.com/vortex-data/vortex/pull/4588).
 1. The work on the new `FixedSizeList` keeps moving forward - [#4590](https://github.com/vortex-data/vortex/pull/4590), [#4601](https://github.com/vortex-data/vortex/pull/4601).