[hotfix][docs] Enforce reading files in UTF-8 in include_without_header.rb
[hotfix][docs] Enforce reading files in UTF-8 in include_without_header.rb

All our documentation pages are encoded in UTF-8. However if not
specified, File.read will use the system default encoding. Therefore
making it impossible to build docs in a system with a different default
encoding. It is the case e.g. when building docs through the docker
container.

darionyaphet/flink,clarkyzl/flink,godfreyhe/flink,clarkyzl/flink,zentol/flink,rmetzger/flink,StephanEwen/incubator-flink,lincoln-lil/flink,zentol/flink,xccui/flink,gyfora/flink,godfreyhe/flink,kl0u/flink,zjureel/flink,tillrohrmann/flink,sunjincheng121/flink,aljoscha/flink,clarkyzl/flink,lincoln-lil/flink,godfreyhe/flink,zentol/flink,xccui/flink,lincoln-lil/flink,tony810430/flink,gyfora/flink,xccui/flink,gyfora/flink,sunjincheng121/flink,zjureel/flink,kl0u/flink,gyfora/flink,darionyaphet/flink,twalthr/flink,zentol/flink,wwjiang007/flink,aljoscha/flink,wwjiang007/flink,rmetzger/flink,kl0u/flink,kl0u/flink,lincoln-lil/flink,rmetzger/flink,lincoln-lil/flink,twalthr/flink,tony810430/flink,greghogan/flink,aljoscha/flink,StephanEwen/incubator-flink,StephanEwen/incubator-flink,zentol/flink,rmetzger/flink,darionyaphet/flink,tillrohrmann/flink,aljoscha/flink,tillrohrmann/flink,wwjiang007/flink,rmetzger/flink,StephanEwen/incubator-flink,zjureel/flink,godfreyhe/flink,godfreyhe/flink,xccui/flink,zjureel/flink,StephanEwen/incubator-flink,greghogan/flink,tony810430/flink,godfreyhe/flink,zjureel/flink,twalthr/flink,godfreyhe/flink,wwjiang007/flink,zentol/flink,apache/flink,lincoln-lil/flink,greghogan/flink,tony810430/flink,apache/flink,greghogan/flink,sunjincheng121/flink,gyfora/flink,zjureel/flink,gyfora/flink,gyfora/flink,wwjiang007/flink,darionyaphet/flink,greghogan/flink,aljoscha/flink,apache/flink,zentol/flink,clarkyzl/flink,xccui/flink,StephanEwen/incubator-flink,apache/flink,tony810430/flink,darionyaphet/flink,xccui/flink,lincoln-lil/flink,wwjiang007/flink,clarkyzl/flink,tillrohrmann/flink,twalthr/flink,tony810430/flink,xccui/flink,tillrohrmann/flink,twalthr/flink,wwjiang007/flink,apache/flink,tillrohrmann/flink,twalthr/flink,zjureel/flink,sunjincheng121/flink,sunjincheng121/flink,rmetzger/flink,apache/flink,tony810430/flink,rmetzger/flink,tillrohrmann/flink,kl0u/flink,greghogan/flink,aljoscha/flink,kl0u/flink,apache/flink,twalthr/flink,sunjincheng121/flink