Add documentation for init.lua

Currently the docs only barely mention the `init.lua` file (where to put it, put this in init.lua to achieve <effect>) but it doesn't have a dedicated page like the other files and it's not really explained what can be done there or what it has access to. One example of what I'd like such documentation to include is that the globals from [yazi-plugin/preset/components](https://github.com/sxyazi/yazi/tree/main/yazi-plugin/preset/components) can be modified, which, in the current documentation, is shown, for example with [`Linemode`](https://yazi-rs.github.io/docs/configuration/yazi/#mgr.linemode), but never explained. A good first step would be a rough overview of `init.lua`'s capabilities and simply pointing people to where they need to look to learn more, especially since there would be significant overlap between a detailed documentation and what the docs for plugins (a beta feature) might have to say about globals. (though I do hope for detailed documentation at some point)