Context-sensitive help

[philrz]

At the time this issue is being opened, Zui is at commit 28aede8.

In a recent discussion between @jameskerr, @mattnibs, and myself, we all agreed that Zui would benefit from some context-sensitive help. This would no doubt he something we'd continue to add going forward in places in Zui that make sense. But since the app doesn't have much of this today, here's some obvious ones we thought might be added to kick things off.

1. Since we already have the minimal syntax highlighting added as part of Monaco Editor In Zui #2824 that recognizes known parts of the language like operators, functions, and aggregate functions, it would be great if we could have tooltips that offer links to the relevant docs page below https://zed.brimdata.io/docs/language for whatever the mouse pointer is being hovered over.

2. It might be helpful to have some hyperlinks (perhaps in a collapsible panel?) alongside the Editor that point to some of the most frequently-referenced areas of the Zed language docs, e.g., Expressions, Search Expressions, Shaping and Type Fusion, Operators, Functions, and Aggregate Functions. A general link to the top-level docs site would probably be worth including here too.

3. A "Zed cheat sheet" (maybe in a similar/same panel?) that lists all the available operators, functions, and aggregate functions with links to their docs. This might save the user the effort of opening up the docs site in a separate browser window if they just need a refresher on what's available or jog their memory as to the name of something that they've used in the past.