fix(ci): hardcode preview site domain fragment

using the repo name, which is itself a domain name, does not work

.github/workflows/site-preview.yml

@@ -56,5 +56,5 @@ jobs:
               issue_number: context.issue.number,
               owner: context.repo.owner,
               repo: context.repo.repo,
-              body: `Preview url: https://${context.repo.repo}-${{ github.event.number }}--${hostnameSuffix}`,
+              body: `Preview url: https://compiler-docs-${{ github.event.number }}--${hostnameSuffix}`,
             })