Zed language "quick reference" doc

[philrz]

@mattnibs recently made the following observation/suggestion:

I spend a lot of time in the language section of the docs and I always get weighed down in the amount of sub-sections. I'm almost always just looking for the list of operators and functions and it always takes me a bit to get my bearings to find it.

A community user recently made a similar observation and also noted they had some difficulty remembering which things are covered by functions vs. operators.

This seems to make the case for some kind of "quick reference" that lists all the functions and operators in a single page with hyperlinks to their relevant docs. Perhaps links to other frequently-referenced language functionality could live at the same page.