[FLINK-12560][Docs] Documentation language build flags

[sjwiesman]

What is the purpose of the change

Most documentation writers are only ever focused on one language at a time. Adding language-specific build flags can dramatically reduce render time during local development.

We add two new flags to the build script
-e: Only build English documentation
-z: Only build Chinese documentation

Verifying this change

Benchmarking on a MacBook Pro

Full clean builds

./build.sh: 176 seconds
./build.sh -e: 96 seconds
./build.sh -z: 62 seconds

Incremental Builds after changing one file
./build.sh -pi : 6.2 seconds
./build.sh -pie : 3.1 seconds

[sjwiesman]

cc @fhueske @wuchong

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[wuchong]

Thanks @sjwiesman , it's a great idea. Could you also add some instruction to docs/README.md ?

[sjwiesman]

@wuchong good call

[wuchong]

Thanks @sjwiesman , looks good to me and I get the same build time in my local machine.

Will merge this.