Allow encoded slash using Apache2 into How-To

[MartinMikita]

According to the IIIF Specification, the image identifier in the @id tag must have encoded slash characters (/ -> %2F).

Using encoded slash character is disabled in Apache 2 by default.
It has to be explicitly allowed, AllowEncodedSlashes, this directive is allowed only in the server config or virtual host context, and IIIFServer installer package cannot set it up.