DOC: schematic of the SSL certificates architecture

This patch provides a schematic of the new architecture based on the
struct cert_key_and_chain which appeared with haproxy 2.1.

Could be backported in 2.1
diff --git a/doc/internals/ssl_cert.dia b/doc/internals/ssl_cert.dia
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index 0000000..f7bde15
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+++ b/doc/internals/ssl_cert.dia
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