lib/crypto: docs: Add rst documentation to Documentation/crypto/

lib/crypto: docs: Add rst documentation to Documentation/crypto/

Add a documentation file Documentation/crypto/libcrypto.rst which
provides a high-level overview of lib/crypto/.

Also add several sub-pages which include the kernel-doc for the
algorithms that have it.  This makes the existing, quite extensive
kernel-doc start being included in the HTML and PDF documentation.

Note that the intent is very much *not* that everyone has to read these
Documentation/ files.  The library is intended to be straightforward and
use familiar conventions; generally it should be possible to dive right
into the kernel-doc.  You shouldn't need to read a lot of documentation
to just call `sha256()`, for example, or to run the unit tests if you're
already familiar with KUnit.  (This differs from the traditional crypto
API which has a larger barrier to entry.)

Nevertheless, this seems worth adding.  Hopefully it is useful and makes
LWN no longer consider the library to be "meticulously undocumented".

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20260418192138.15556-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>