docs: Add documentation for llext

Adds a basic overview and API docs for llext.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>

doc/develop/api/overview.rst

@@ -201,6 +201,10 @@ between major releases are available in the :ref:`zephyr_release_notes`.
      - Unstable
      - 1.9
 
+   * - :ref:`llext`
+     - Experimental
+     - 3.5
+
    * - :ref:`logging_api`
      - Stable
      - 1.13

doc/services/index.rst

@@ -16,6 +16,7 @@ OS Services
    formatted_output.rst
    input/index.rst
    ipc/index.rst
+   llext/index.rst
    logging/index.rst
    tracing/index.rst
    resource_management/index.rst

doc/services/llext/index.rst

@@ -0,0 +1,30 @@
+.. _llext:
+
+Linkable Loadable Extensions (LLEXT)
+####################################
+
+The llext subsystem provides a toolbox for extending the functionality of an
+application at runtime with linkable loadable code.
+
+Extensions can be loaded from precompiled ELF formatted data which is
+verified, loaded, and linked with other extensions. Extensions can be
+manipulated and introspected to some degree, as well as unloaded when no longer
+needed.
+
+An extension may be loaded using any implementation of a :c:struct:`llext_loader`
+which has a set of function pointers that provide the necessary functionality
+to read the ELF data. A loader also provides some minimal context (memory)
+needed by the :c:func:`llext_load` function. An implementation over a buffer
+containing an ELF in addressable memory in memory is available as
+:c:struct:`llext_buf_loader`.
+
+API Reference
+*************
+
+.. doxygengroup:: llext
+
+.. doxygengroup:: llext_symbols
+
+.. doxygengroup:: llext_loader
+
+.. doxygengroup:: llext_buf_loader