IPv6 corrections (#628)

* Update unsupported features list in ipv6.rst

Remove the security group from IPv6 limitations

* Update IPv6 support information in documentation

Clarified IPv6 support details for shared networks, including deployment models and limitations.

* IPv6 page formatting

* Update IPv6 Shared Network documentation with address assignment and routing details

* Update IPv6 support

* Clarify that IPv6 Start IP and End IP are optional parameters

* IPv6: fix heading level for IPv6 Address Assignment and Routing section

Use the subsection (#) underline instead of the page-title (=) underline so
the section nests correctly under 'Shared network' instead of rendering as a
top-level sibling that detaches the following sections.