doc: move images into their own subdirectory

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15399)

diff --git a/doc/man7/kdf.png b/doc/man7/img/kdf.png
similarity index 100%
rename from doc/man7/kdf.png
rename to doc/man7/img/kdf.png

diff --git a/doc/man7/mac.png b/doc/man7/img/mac.png
similarity index 100%
rename from doc/man7/mac.png
rename to doc/man7/img/mac.png

diff --git a/doc/man7/rand.png b/doc/man7/img/rand.png
similarity index 100%
rename from doc/man7/rand.png
rename to doc/man7/img/rand.png