Move the description of the core types into their own pages

This expands on some of the core type descriptions, and also makes it
easier to find the documentation for each type, at least

Move the description of the core types into their own pages

This expands on some of the core type descriptions, and also makes it
easier to find the documentation for each type, at least on Unix, with
a simple call like "man OSSL_ALGORITHM".

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19842)

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