Move prerelease-subdomain from phase profiles to base/site profiles

prerelease-subdomain is a site concern (which domain am I on?), not a
phase concern (prerelease vs RC). Previously, the rc profile set it to
empty and the prerelease profile set it to 'prerelease.', which caused
the main site to link to prerelease.quarto.org during dev phase.

Now the base _quarto.yml defines prerelease-subdomain as empty (links
to quarto.org) and only _quarto-prerelease-docs.yml overrides it to
'prerelease.' for the prerelease site.

Author: cderv

_quarto-prerelease-docs.yml

@@ -1,6 +1,8 @@
 # Pre-release version number
 version: v1.9
 
+prerelease-subdomain: prerelease.
+
 website:
   title: "Quarto (Pre-release)"
   image: "quarto-dark-bg-pre.jpeg"

_quarto-prerelease.yml

@@ -1,4 +1,3 @@
 prerelease-title: Pre-release
 prerelease-lower: pre-release
-prerelease-mode: ""
-prerelease-subdomain: prerelease.
+prerelease-mode: ""

_quarto-rc.yml

@@ -1,4 +1,3 @@
 prerelease-title: Release Candidate
 prerelease-lower: release candidate
-prerelease-mode: Release Candidate
-prerelease-subdomain: ''
+prerelease-mode: Release Candidate

_quarto.yml

@@ -706,6 +706,8 @@ filters:
   - at: post-quarto
     path: filters/include-dark.lua
 
+prerelease-subdomain: ''
+
 freeze: true
 
 profile: