Added documentation for ImageMorph module

[johnnygitgud]

This pull request adds documentation for the ImageMorph module
For issue #713 ("Todo: Docs for ImageMorph").

The updated documentation has:

• Overview of morphological operations in Pillow
• Explanation of structuring element patterns and ASCII masks
• Usage examples for LutBuilder and MorphOp
• Examples demonstrating pattern matching and apply()
• Information about LUT loading, saving, and reuse
• Guidance on required image modes

[hugovk]

Thank you for filling this gap in the docs!

---

Please can you check the Sphinx warnings?

https://app.readthedocs.org/projects/pillow/builds/30336202/?utm_source=pillow&utm_content=notification#294395027--277

ImageMorph.rst:13: WARNING: py:class reference target not found: LutBuilder [ref.class]
ImageMorph.rst:13: WARNING: py:class reference target not found: MorphOp [ref.class]
ImageMorph.rst:54: WARNING: py:class reference target not found: LutBuilder [ref.class]
ImageMorph.rst:59: WARNING: py:class reference target not found: LutBuilder [ref.class]
ImageMorph.rst:62: WARNING: py:class reference target not found: LutBuilder [ref.class]
ImageMorph.rst:83: WARNING: py:meth reference target not found: LutBuilder.get_lut [ref.meth]
ImageMorph.rst:83: WARNING: py:meth reference target not found: LutBuilder.load_lut [ref.meth]
ImageMorph.rst:83: WARNING: py:meth reference target not found: LutBuilder.save_lut [ref.meth]
ImageMorph.rst:87: WARNING: py:class reference target not found: MorphOp [ref.class]
ImageMorph.rst:90: WARNING: py:class reference target not found: MorphOp [ref.class]
ImageMorph.rst:90: WARNING: py:meth reference target not found: MorphOp.apply [ref.meth]
ImageMorph.rst:109: WARNING: py:meth reference target not found: MorphOp.match [ref.meth]
ImageMorph.rst:109: WARNING: py:meth reference target not found: MorphOp.get_on_pixels [ref.meth]
ImageMorph.rst:125: WARNING: py:class reference target not found: LutBuilder [ref.class]
ImageMorph.rst:145: WARNING: py:mod reference target not found: _imagingmorph [ref.mod]
ImageMorph.rst:145: WARNING: py:class reference target not found: LutBuilder [ref.class]
ImageMorph.rst:145: WARNING: py:class reference target not found: MorphOp [ref.class]

[radarhere]

This seems redundant, as values in L mode images only range from 0 to 255.