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 #22029)

doc/designs/prov_loadex.md → doc/designs/ossl-provider-load-ex.md

@@ -1,5 +1,5 @@
-Providers run-time configuration
-================================
+OSSL_PROVIDER_load_ex - activating providers with run-time configuration
+========================================================================
 
 Currently any provider run-time activation requires the presence of the
 initialization parameters in the OpenSSL configuration file. Otherwise the