Merge PFM documentation into PPM

[radarhere]

Most of the headings at https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html match the format attribute, as used by open() or save().

This updates it so that they all do.

While here, I have also updated the format / id for PalmImagePlugin to be uppercase, since Image immediately transforms it anyway.

Pillow/src/PIL/PalmImagePlugin.py

Lines 213 to 217 in 913698b

	Image.register_save("Palm", _save)
	Image.register_extension("Palm", ".palm")
	Image.register_mime("Palm", "image/palm")

Pillow/src/PIL/Image.py

Lines 3845 to 3855 in 913698b

	def register_save(
	id: str, driver: Callable[[Image, IO[bytes], str | bytes], None]
	) -> None:
	"""
	Registers an image save function. This function should not be
	used in application code.
	:param id: An image format identifier.
	:param driver: A function to save images in this format.
	"""
	SAVE[id.upper()] = driver

Pillow/src/PIL/Image.py

Lines 3872 to 3880 in 913698b

	def register_extension(id: str, extension: str) -> None:
	"""
	Registers an image extension. This function should not be
	used in application code.
	:param id: An image format identifier.
	:param extension: An extension used for this format.
	"""
	EXTENSION[extension.lower()] = id.upper()

Pillow/src/PIL/Image.py

Lines 3830 to 3842 in 913698b

	def register_mime(id: str, mimetype: str) -> None:
	"""
	Registers an image MIME type by populating ``Image.MIME``. This function
	should not be used in application code.
	``Image.MIME`` provides a mapping from image format identifiers to mime
	formats, but :py:meth:`~PIL.ImageFile.ImageFile.get_format_mimetype` can
	provide a different result for specific images.
	:param id: An image format identifier.
	:param mimetype: The image MIME type for this format.
	"""
	MIME[id.upper()] = mimetype

[hugovk]

I think the "human readable" names, where available, are more, well, readable.

[radarhere]

Does it at least make sense to combine the PFM and PPM entries? They are handled by one plugin.