Update the external links in the doc index

Splits 'extern resources' into 'external documentation and 'community',
adds rustforrubyists.com and rustbyexample.com, stackoverflow.com.
doc: Add disclaimers about versions of external docs
auto merge of #15041 : brson/rust/docindex, r=alexcrichton

Splits 'extern resources' into 'external documentation and 'community',
adds rustforrubyists.com, rustbyexample.com, stackoverflow.com.