The new ActionHooks docs had some section headers in page title form,

and some other section level ordering issues. These prevented Sphinx

from properly determining the section levels, and that resulted in some

references failing.
Along with this, Sphinx wasn't successfully linking to the 6.x

development docs at all. This was due to both the rb6.x and rb-dev

intersphinx names pointing to the same URL, which failed.
This change fixes up the section header levels and removes the (unused)

rb-dev intersphinx name.

Successfully built and uploaded new docs and release notes without any

linking issues.