MAINT: Reduce Pixi RAM usage in RTD (#11177)

Fixes #11175

Author: VeckoTheGecko

.readthedocs.yaml

@@ -18,7 +18,7 @@ build:
       - (git --no-pager log --pretty="tformat:%s" -1 | grep -vqF "[skip-rtd]") || exit 183
       - git fetch --unshallow || true
     install:
-      - pixi install -e doc
+      - pixi install --concurrent-solves 2 -e doc
     build:
       html:
         - pixi run doc BUILDDIR=$READTHEDOCS_OUTPUT