readthedocs: use uv to build instead of pip

`uv` is faster than `pip`, and `pip install` can take a while in our
readthedocs builds in CI. Try using `uv` instead.

See [readthedocs's docs](https://docs.readthedocs.com/platform/stable/build-customization.html#install-dependencies-with-uv)
for details.

Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>

.readthedocs.yml

@@ -6,6 +6,17 @@ build:
     - graphviz
   tools:
     python: "3.11"
+  jobs:
+    create_environment:
+      - asdf plugin add uv
+      - asdf install uv latest
+      - asdf global uv latest
+      - uv venv
+    install:
+      - uv pip install -r requirements.txt
+    build:
+      html:
+        - uv run sphinx-build -T -b html docs $READTHEDOCS_OUTPUT/html
 
 sphinx:
   configuration: lib/spack/docs/conf.py