doc: avoid python.system_packages=true in .readthedocs.yaml

Apparently python.system_packages=true is problematic in the RTD
environment [1]. Install libclang-14-dev from apt and clang from pip,
using specific versions for both.

Attempt to install clang using build.jobs.post_install instead of a
requirements.txt to avoid defining the version in two different
files. Fingers crossed.

[1] readthedocs/readthedocs.org#10500

Author: jnikula

.readthedocs.yaml

@@ -6,13 +6,15 @@ build:
   tools:
     python: "3.11"
   apt_packages:
-    - python3-clang
+    - libclang-14-dev
+  jobs:
+    post_install:
+      - python -m pip install --upgrade --no-cache-dir clang==14.0.6
 
 python:
   install:
     - method: pip
       path: .
-  system_packages: true
 
 formats: all