Add keyword index and list of related keywords

[pauljurczak]

Following my post https://discourse.julialang.org/t/how-to-capture-a-for-loop-index-after-break/46342 I found that outer keyword is not systematically discoverable from Julia documentation. Search for outer produces 110 results with the relevant page almost at the end and hard to identify as such. JuliaHub does a bit better, but still no direct reference to the paragraph, where outer is defined.

One possible solution to this problem would be a keyword index with links to chapters or paragraphs where given keyword is defined. In addition, a list of related keyword could be provided, e.g. for would have outer, break and end related keywords listed.