docs: Build RTD artifacts directly

Use rtds-action instead of yosys-cmd-ref repo.
Add rtds_action to docs configuration.
Add `.readthedocs.yaml`.
Update `DOCS_USAGE_` make target to be able to use pre-generated executables without forcing a remake.

.readthedocs.yaml

@@ -0,0 +1,19 @@
+# .readthedocs.yaml
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+version: 2
+
+build:
+    os: ubuntu-22.04
+    tools:
+        python: '3.12'
+
+formats:
+  - pdf
+
+sphinx:
+  configuration: docs/source/conf.py
+
+python:
+  install:
+    - requirements: docs/source/requirements.txt