Provide a single page index of all Dolt extension functions/procedures/variables/tables

[fulghum]

A customer requested a single page index of all Dolt functions, procedures, system tables, system variables, etc.

Today, this information is documented on our doc site, but there isn't a single page where you can see it all together. If you know the name of what you're looking for, you can use the site's search box to find it, and if you know whether it's a function/procedure/etc you can go directly to those pages. This single page index would be useful if you don't know the name or the type, and just want to browse all the Dolt extensions in one page and search for keywords to find something.

[fulghum]

We added a Dolt cheat sheet that I think meets the intent of this request – it doesn't require you to know how functionality is exposed in order to find a feature you're looking for. For example, if you're looking for diff functionality, you don't have to know that the diff features are exposed as table functions and go look at the functions page – instead, you can search for "diff" on the Dolt cheat sheet and get a quick overview of the diff support, along with links to deeper documentation.