ci(rtd): docs-only build — drop pip install . and openblas apt dep

RTD builds were failing because openblas-build 0.10.16 (transitively pulled by
ndarray-linalg via openblas-src) requires ureq tls feature flags it does not
enable. Cargo.lock is gitignored so RTD always resolves fresh and hits the
broken upstream version.

Building the Rust extension on RTD is unnecessary: the Sphinx pages do not
use autodoc against the Python module, and all inline plots under doc samples
are pre-rendered PNGs committed to docs/source/_static/auto/ by the local
scripts/inject_doc_plots.py author-time pipeline.

This commit makes the RTD job render-only: install docs/requirements.txt and
run Sphinx. No Rust toolchain, no maturin, no OpenBLAS.

.readthedocs.yaml

@@ -7,8 +7,6 @@ build:
   os: ubuntu-22.04
   tools:
     python: "3.11"
-  apt_packages:
-    - libopenblas-dev
 
 sphinx:
   configuration: docs/source/conf.py
@@ -20,5 +18,9 @@ formats:
 python:
   install:
     - requirements: docs/requirements.txt
-    - method: pip
-      path: .
+# Note: we intentionally do NOT `pip install .` on RTD.
+# The Sphinx build only renders RST/MD pages and pre-generated PNG plots
+# committed under docs/source/_static/. Building the Rust extension on RTD
+# requires OpenBLAS + maturin and routinely breaks because of upstream
+# `openblas-build` / `ureq` feature changes. All inline plots are pre-rendered
+# locally via scripts/inject_doc_plots.py and committed to the repo.