docs: add comprehensive documentation structure and API reference

Created topic-based documentation:
- basics.rst: core concepts and fundamental usage
- cyclic_programming.rst: PLC-style programming with Cycletools
- event_programming.rst: event-driven patterns and callbacks
- advanced.rst: gateway IOs, replace_io_file, watchdog management
- installation.rst and quickstart.rst: getting started guides

Added complete API reference in docs/api/:
- All device classes including ModularBaseConnect_4_5 and GatewayMixin
- I/O, helper, and main class documentation

Enhanced Sphinx configuration with RTD theme and improved autodoc settings.
Removed auto-generated modules.rst and revpimodio2.rst.

docs/index.rst

@@ -1,18 +1,87 @@
-.. revpimodio2 documentation main file, created by
-   sphinx-quickstart on Sun Jan 22 17:49:41 2023.
-   You can adapt this file completely to your liking, but it should at least
-   contain the root `toctree` directive.
+====================================
+Welcome to RevPiModIO Documentation!
+====================================
 
-Welcome to revpimodio2's documentation!
-=======================================
+RevPiModIO is a Python3 module for programming Revolution Pi hardware from KUNBUS GmbH.
+It provides easy access to all devices and IOs from the piCtory configuration, supporting
+both cyclic (PLC-style) and event-driven programming paradigms.
+
+.. note::
+   **New to RevPiModIO?** Start with :doc:`installation` and :doc:`quickstart`.
+
+Key Features
+============
+
+* **Dual Programming Models**: Cyclic (PLC-style) and event-driven approaches
+* **Direct Hardware Access**: Simple Python interface to all I/O devices
+* **Automatic Configuration**: Loads piCtory device configuration
+* **Event System**: Callbacks for value changes and timer events
+* **Open Source**: LGPLv2 license, no licensing fees
+
+Quick Example
+=============
+
+**Cyclic Programming**::
+
+    import revpimodio2
+    rpi = revpimodio2.RevPiModIO(autorefresh=True)
+
+    def main(ct):
+        if ct.io.button.value:
+            ct.io.led.value = True
+
+    rpi.cycleloop(main)
+
+**Event-Driven Programming**::
+
+    import revpimodio2
+    rpi = revpimodio2.RevPiModIO(autorefresh=True)
+
+    def on_change(ioname, iovalue):
+        print(f"{ioname} = {iovalue}")
+
+    rpi.io.button.reg_event(on_change)
+    rpi.handlesignalend()
+    rpi.mainloop()
+
+Documentation
+=============
 
 .. toctree::
    :maxdepth: 2
-   :caption: Contents:
+   :caption: Getting Started
 
+   installation
+   quickstart
 
+.. toctree::
+   :maxdepth: 2
+   :caption: User Guide
 
-Indices and tables
+   basics
+   cyclic_programming
+   event_programming
+   advanced
+
+.. toctree::
+   :maxdepth: 3
+   :caption: API Reference
+
+   api/index
+   api/revpimodio
+   api/io
+   api/helper
+   api/device
+
+External Resources
+==================
+
+* **Official Website**: `revpimodio.org <https://revpimodio.org>`_
+* **GitHub Repository**: `github.com/naruxde/ <https://github.com/naruxde/>`_
+* **Discussion Forum**: `revpimodio.org/diskussionsforum/ <https://revpimodio.org/diskussionsforum/>`_
+* **Revolution Pi Hardware**: `revolution.kunbus.com <https://revolution.kunbus.com>`_
+
+Indices and Tables
 ==================
 
 * :ref:`genindex`