GitLab

 - clarified JSON usage in admin guide (True/False not allowed)
 - fixed incorrect reference to hosts4-storage
 - clarified that empty mac-sources entry is not allowed
 - added extra text about syntactic contexts
 - corrected several typos

- Extended description of the S46 Rule Option
- Added Lightweight 4over6 to the softwire examples
- Updated Supported DHCPv6 standards with RFC6334
- Updated DHCPv6 spec file

This change includes the code implemented by the pull request
submitter as well as Marcin's fixes/changes on top of it, but
without a review.

Removed a todo, updated example with multiple options and added
item descriptions in the DHCPv6 spec file.

Move Kea logo to left of title in the header.  Also fix an incorrect
cross-reference link in the DHCPv4 chapter.

Updated:
- Supported OSes
- Packages required at runtime to run Kea,
- Packages required to build Kea