docs: remove Contents panel

[vinniefalco]

I would be better to leave out the "Contents" panel on the right side of the Antora documentation. I believe this is done in the playbook or perhaps in the main adoc file (@alandefreitas?)

[alandefreitas]

There's an option to remove the side nav on the left from the UI:

https://github.com/boostorg/website-v2-docs/blob/c07d3e282c69cb7065b99f6671bfc6e445183ef1/libs.playbook.yml#L68

It's currently enabled for libs.playbook.yml and disabled for site.playbook.yml. We can change this combination at any time.

There's no option for the contents panel on the right. IIRC, the guys working on the CSS were asked to delete it, and anything left behind is a bug.

[vinniefalco]

Yes we should remove it completely