doc: Add ulib documentation for shared and static libraries

Add documentation for the U-Boot Library (ulib) feature, explaining how
to build and use both shared (.so) and static (.a) libraries.

Series-to: concept
Cover-letter:
ulib: Provide test programs and documentation
This series completes the implementation of a basic U-Boot library:

- ensure that no output is written during init
- provide a static library to match the existing shared library
- add test programs for both, to ensure they continue to build correctly
- provide documentation to describe how to use the libraries

This is still very early in the implementation, but this series provides
enough for some initial experimentation.
END

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Series-links: 1:25

doc/develop/index.rst

@@ -56,6 +56,7 @@ Implementation
    falcon
    std_passage
    uefi/index
+   ulib
    vbe
    version