doc: split buffer.rst out of api-summary.rst

doc: split buffer.rst out of api-summary.rst

Buffer heads are no longer a generic filesystem API but an optional
filesystem support library.  Make the documentation structure reflect
that, and include the fine documentation kept in buffer_head.h.  We could
give a better overview of what buffer heads are all about, but my
enthusiasm for documenting it is limited.

[willy@infradead.org: fix kerneldoc warning]
Link: https://lkml.kernel.org/r/20240417015933.453505-1-willy@infradead.org
[akpm@linux-foundation.org: remove newline at EOF]
Link: https://lkml.kernel.org/r/20240416031754.4076917-9-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>