docs: v2.12.0 changelog

Author: kozistr

docs/changelogs/v2.12.0.md

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+## Change Log
+
+### Feature
+
+* Support `bitsandbytes` optimizer. (#211)
+    * now, you can install with `pip3 install pytorch-optimizer[bitsandbytes]`
+
+### Docs
+
+* Introduce `mkdocs` with `material` theme.
+    * documentation : https://pytorch-optimizers.readthedocs.io/en/latest/
+
+### Diff
+
+[2.11.2...2.12.0](https://github.com/kozistr/pytorch_optimizer/compare/v2.11.2...v2.12.0)