Add prerelease-link-subdomain for phase-aware linking to prerelease docs

Blog posts on main need to link to prerelease.quarto.org during RC phase
(when docs only exist there) and switch to quarto.org after release.

Unlike prerelease-subdomain (site identity), this variable is phase-aware:
- Default: '' (links to quarto.org)
- RC profile: 'prerelease.' (docs still on prerelease site)
- prerelease-docs profile: 'prerelease.' (prerelease site links to itself)

Author: cderv

README.md

@@ -132,7 +132,16 @@ The group order determines which phase is active on **quarto.org** (the main sit
 |---|---|
 | `_quarto-prerelease-docs.yml` | Site-specific configuration for prerelease.quarto.org |
 
-The `prerelease-subdomain` variable is defined as empty (`''`) in `_quarto.yml` (so links point to `quarto.org` by default) and overridden to `prerelease.` only in `_quarto-prerelease-docs.yml`.
+### Subdomain variables
+
+Two variables control how links resolve across builds. Both use the same pattern — `https://{{< meta VAR >}}quarto.org/...` — but serve different purposes:
+
+| Variable | Purpose | Default | Set by `rc` | Set by `prerelease-docs` |
+|---|---|---|---|---|
+| `prerelease-subdomain` | **Site identity** — "am I the prerelease site?" | `''` | — | `prerelease.` |
+| `prerelease-link-subdomain` | **Content linking** — "where do prerelease docs live right now?" | `''` | `prerelease.` | `prerelease.` |
+
+Use `prerelease-subdomain` for self-referential links (e.g. RevealJS demo links back to its own site). Use `prerelease-link-subdomain` for content on `main` that references docs only available on prerelease during RC phase (e.g. blog posts announcing upcoming features).
 
 ### Release lifecycle
 

_quarto-prerelease-docs.yml

@@ -2,6 +2,7 @@
 version: v1.9
 
 prerelease-subdomain: prerelease.
+prerelease-link-subdomain: prerelease.
 
 website:
   title: "Quarto (Pre-release)"

_quarto-rc.yml

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

_quarto.yml

@@ -707,6 +707,7 @@ filters:
     path: filters/include-dark.lua
 
 prerelease-subdomain: ''
+prerelease-link-subdomain: ''
 
 freeze: true