Add support for elements to be added to the sidebar (and navbar?)

[Badgerati]

This will need to investigation first, mostly for the navbar, but it would be nice to have support added for elements to be added to the sidebar/navbar.

ie:

Edit-PodeWebSidebar -PreContent @(
    New-PodeWebButton -Name 'Do The Thing!' -Scriptblock { ... }
)

I'm not sure properly how this will work, possibly like above we have a -PreContent and -PostContent which adds elements before/after the normal sidebar navigation 🤔 and from #435 we could add a -Content parameter to New-PodeWebPageGroup to further add custom element within the collapsible groupings.