Add a useful customizations page

[krader1961]

Introduce a new Elvish document that suggests changes to a user's rc.elv likely to be useful by most Elvish users. There are a large number of other customizations I rely on but most of them are too niche to include in a document like this one. The intent is to introduce a page that documents customizations that a significant percentage of Elvish users are likely to find useful and adopt. Arguably this could be a FAQ page but it seems to me a page devoted to commonly used customizations is distinct from the sort of information typically found on a FAQ.