doc: change name of OSSL_PROVIDER_load_ex design document

doc: change name of OSSL_PROVIDER_load_ex design document

Use dashes instead of underscores, to be more consistent with
existing document names. And speaking of consistency, introduce
a consistent name transformation, which will scale better when
design documents start filling the folder ;-)

    OSSL_PROVIDER_load_ex -> ossl-provider-load-ex.md

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22029)