docs(modem): Add migration manual

Author: david-cermak

docs/esp_modem/en/index.rst

@@ -4,6 +4,7 @@ ESP-MODEM Programmers manual
 .. toctree::
 
    Brief intro <README>
+   Migration Guide <migration_guide>
    C interface <api_docs>
    C++ interface <cxx_api_docs>
    Advanced use cases <advanced_api>

docs/esp_modem/en/migration_guide.rst

@@ -0,0 +1,64 @@
+Migration Guide
+===============
+
+ESP-MODEM v2.0 introduces production mode (default) with pre-generated sources for better IDE navigation. Previous behavior (development mode) requires explicit configuration.
+
+Breaking Changes
+----------------
+
+**Production Mode (Default)**
+- Uses pre-generated sources from ``command/`` directory
+- Better IDE navigation and code completion
+- Faster compilation
+
+**Development Mode (Optional)**
+- Uses macro expansion from ``generate/`` directory
+- Enable with ``CONFIG_ESP_MODEM_ENABLE_DEVELOPMENT_MODE=y``
+- Required for modifying core ESP-MODEM files
+
+Migration Steps
+---------------
+
+**Application Developers:**
+No changes required. Production mode is default.
+
+**Library Developers:**
+Enable development mode:
+
+.. code-block:: bash
+
+    idf.py -D CONFIG_ESP_MODEM_ENABLE_DEVELOPMENT_MODE=y build
+
+**Custom ``*.inc`` Files:**
+Use generation script:
+
+.. code-block:: bash
+
+    ./components/esp_modem/scripts/generate.sh your_file.inc
+
+**Build:**
+.. code-block:: bash
+
+    idf.py fullclean
+    idf.py build
+
+New Features (Coming Soon)
+--------------------------
+
+**Better URC Handling**
+- Enhanced unsolicited result code processing
+- Existing URC code remains compatible
+
+**AT-based Networking**
+- AT command networking examples now supports multiple connections
+
+Troubleshooting
+---------------
+
+**Build errors:** ``idf.py fullclean && idf.py build``
+
+**No IDE completion:** Use production mode (default)
+
+**Custom .inc files:** Use ``./components/esp_modem/scripts/generate.sh your_file.inc``
+
+**Modify core files:** Enable ``CONFIG_ESP_MODEM_ENABLE_DEVELOPMENT_MODE=y``