docs: support items without nav sidebar entry

[dashohoxha]

In some cases it might be useful to hide (not-show) a page on the left-side menu.
For example in the "Installation" page, the pages about "completion" and "plugins" are referenced from the index page of installation, but I wish they were not shown on the sidebar menu.

Currently this is not possible (unless I have missed something).
This could be specified for example with a field like "label": false, or "label": null, or "label": "",

[jorgeorpinel]

Seems to me like a correct feature: being able to have the files there for linking, but with no entry in sidebar.json (Basically, content hidden from navigation)

However I don't agree that neither /doc/user-guide/install/completion nor ...plugins should be hidden from the sidebar. They help users discover those topics by navigating the side bar. In fact can't find any element in the navigation at the moment that shouldn't be listed.

[jorgeorpinel]

p.s. The problem with those 2 entries is that they are repeated as stand-alone User Guides since #695 was merged. This is reported in #701.

[shcheklein]

Not sure I see any valuable use case for this for now. Closing this. Will revisit if have a real use case for this.