Make Unbound installation docs clearer

Problem to solve

As Gus suggested, the subpages for Unbound are missing clarification about what the subpage is about (if directly accessed, and not after reading the exit docs).

Furthermore, the URL may be a bit deceiving. Right now it is https://community.torproject.org/relay/setup/exit/debian-ubuntu/. Maybe creating another parent directory (.../relay/setup/exit/dns-resolver/$os/) or renaming the current directories to something like unbound-debian-ubuntu or dns-debian-ubuntu may be more appropriate. Let me know what you think.

Proposal

My main proposal is to modify the appropriate lektor template, so the subtitle is also shown on the subpage header, right under the main title and with a smaller font size. Since many subpages already use the 'subtitle' parameter, no major changes would be needed. However, I wonder if this change would negatively affect any other documentation pages. As a downside, the OS name gets repeated if stated on the subtitle, not as good looking as it should and not sure if it's good for accessibility.

If this is a no go, I will happily submit a MR with new phrases (disclaimers?) to make the matter clearer.

Who can address the issue

Anyone with a better understanding of the lektor structure being used than I do.

Other links/references

Related to !367 (merged)