feat: sub item documentation for completion menus

[NikitaRevenco]

Typed Commands show a little box containing information about the command:

Wouldn't it also be nice if sub-commands also contained information boxes? Currently, you don't get any documentation beyond the first command:

I added this ability. Now, you have documentation per-option directly within the editor:

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I only added a few entries for documentation as this is a proof-of-concept. Some ideas to consider:

• We could store the documentation in markdown and display it here. Since Helix can display Markdown, it would have code blocks, syntax highlighting and all
• This is a bare implementation. Thoughts on this idea maintainers?
• We could store all documentation this way and then extend cargo xtask to generate the website's documentation. I'm happy to port the current website's e.g. editor reference right into the editor. This is just one possibility.

Thoughts on this PR? I personally really wanted this feature because I think having documentation within the editor more accessible is always a good idea. Let me know in the comments

[gabydd]

This is a nice idea one thing we probably want before this is #997 then we can add the config option documentation to in editor help as well