Fix broken build configs: .readthedocs.yaml and Dockerfile (#594)

Fix .readthedocs.yaml referencing non-existent requirements-dev.txt by
using pip install with doc extras from pyproject.toml instead.

Update Dockerfile to remove unnecessary C snap7 library dependencies
(libsnap7-dev, libsnap7-1) since python-snap7 3.0 is pure Python.
Update image description label.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: gijzelaerr

.readthedocs.yaml

@@ -12,4 +12,8 @@ sphinx:
 
 python:
   install:
-    - requirements: requirements-dev.txt
+    - method: pip
+      path: .
+      extra_requirements:
+        - doc
+        - cli

Dockerfile

@@ -2,14 +2,11 @@ FROM ubuntu:24.04
 ENV DEBIAN_FRONTEND=noninteractive
 
 LABEL org.opencontainers.image.source=https://github.com/gijzelaerr/python-snap7
-LABEL org.opencontainers.image.description="The snap7 library is used to communicate with Siemens S7 PLCs. This is a Python wrapper for the snap7 library."
+LABEL org.opencontainers.image.description="Pure Python S7 communication library for interfacing with Siemens S7 PLCs."
 LABEL org.opencontainers.image.licenses=MIT
 
 RUN apt update \
-    && apt install -y software-properties-common python3-pip python3-venv \
-    && add-apt-repository ppa:gijzelaar/snap7 \
-    && apt update \
-    && apt install -y libsnap7-dev libsnap7-1
+    && apt install -y python3-pip python3-venv
 ADD . /code
 WORKDIR /venv
 RUN python3 -m venv /venv