[org] add org-sidebar package to org layer

[wolfand11]

Feature request

add the https://github.com/alphapapa/org-sidebar package to the org layer.

org-sidebar-tree can display tree-view sidebar for current Org buffer.

The package is available via MELPA https://melpa.org/#/org-sidebar

Thank you.

[emacs18]

treemacs-mode provides this feature as well not only for org-mode, but many other files as well.

[ryanprior]

@wolfand11 I'd be happy to pair with you on a patch for this. Personally I'm happy with Treemacs as suggested by @emacs18 but adding org-sidebar-tree as an option should be straightforward!