Update RTD config to use 'uv' for faster builds

Using `build.commands`, we completely override the build process for
RTD. Per their documentation (linked in a comment), this is the
way to convert a doc build to use `uv`.

The install is done using the `uv pip` interface, which has a few
advantages:
- it can be treated as a plug-in replacement for `pip ...`
- it supports `-r` options (so we don't need to change how we're
  locking dependencies here)
- it ensures that we don't see any surprising new semantics from the
  uv lockfile (which could be hard to troubleshoot locally)

Because we're now using `build.commands`, there's no point in keeping
`jobs.pre_build` as separate config. Simply inline the relevant
command into our overridden process.

.readthedocs.yml

@@ -8,12 +8,12 @@ build:
   os: "ubuntu-22.04"
   tools:
     python: "3.11"
-  jobs:
-    pre_build:
-      - bash ./scripts/rtd-pre-sphinx-build.sh
-
-python:
-  install:
-    - method: pip
-      path: .
-    - requirements: "requirements/py3.11/docs.txt"
+  # use 'uv pip install' to apply build customization in a pip-compatible way
+  # see: https://docs.readthedocs.io/en/stable/build-customization.html#install-dependencies-with-uv
+  commands:
+    - bash ./scripts/rtd-pre-sphinx-build.sh
+    - asdf plugin add uv
+    - asdf install uv latest
+    - asdf global uv latest
+    - uv pip install . -r "requirements/py3.11/docs.txt"
+    - uv run -m sphinx -T -b html -d docs/_build/doctrees -D language=en docs $READTHEDOCS_OUTPUT/html