build: rtd: install asciidoc via pip

The rtb build fails because the something is depending on the asciidoc
python module which is missing. Since sphinx is installed via the pip
step anyway, move it to the pip install section.

Also drop the unnecessary dependencies.

Signed-off-by: Daniel Wagner <[email protected]>

Author: igaw

.readthedocs.yaml

@@ -7,12 +7,11 @@ build:
     python: "3.14"
   apt_packages:
     - meson
-    - python3-lxml
-    - docbook-xsl
-    - xsltproc
-    - pandoc
-    - asciidoc
-    - python3-sphinx
+  python:
+    install:
+      - pip:
+          - asciidoc
+          - sphinx
   jobs:
     build:
       html: