Consolidate docs for vacuum-related GUCs in new subsection

Consolidate docs for vacuum-related GUCs in new subsection

GUCs related to vacuum's freezing behavior were documented in a
subsection of the Client Connection Defaults documentation. These GUCs
don't belong there, as they affect the freezing behavior of all vacuums
-- including autovacuums.

There wasn't a clear alternative location, so this commit makes a new
"Server Configuration" docs subsection, "Vacuuming", with a subsection
for "Freezing". It also moves the "Automatic Vacuuming" subsection and
the docs on GUCs controlling cost-based vacuum delay under the new
"Vacuuming" subsection.

The other vacuum-related GUCs under the "Resource Consumption"
subsection have been left in their current location, as they seem to fit
there.

The GUCs' documentation was largely lifted and shifted. The only
modification made was the addition of a few missing <literal> tags.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/flat/CAAKRu_aQUOaMYrcjNuXeSkJtaX9oRUzKP57bsYbC0gVVWS%2BcbA%40mail.gmail.com