chore: point the placeholder homepage at the org wiki

The Cargo homepage feeds Plugin::URL, which hosts can show to users, so
the placeholder should be somewhere reachable rather than a github.com URL
that never existed.

This retires YOUR-USERNAME and YOUR-PLUGIN-NAME as canonical values, which
is the self-arming STALE mechanism working as designed: both were skipped
while they were part of HOMEPAGE and became live checks the moment they
were not. Both pass, since the whole URL was replaced.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WHt4DykxXaQnyaJfyS8KCn

.github/workflows/ci.yaml

@@ -58,4 +58,4 @@ jobs:
       # not exercise — the bundle step is where bundler.toml and the crate name
       # have to agree.
       - name: Bundle
-        run: cargo xtask bundle nih_plug_template --release --features vst3
+        run: cargo xtask bundle plugin_name --release --features vst3