docs: fix gitlab pages deploy syntax

[jonthesquirrel]

GitLab CI fails with this error when building the site using the .gitlab-ci.yml example in the vitepress docs.
error [email protected]: The engine "node" is incompatible with this module. Expected version ">=14.0.0". Got "10.22.0"

This change will:

• Use a compatible node version
• Use a conditional rule instead of a hardcoded branch name
• Mark it as the deploy stage

The last two changes are for consistency with a similar .gitlab-ci.yml example in the main vite docs:

• docs: fix gitlab pages deploy syntax vitejs/vite#5909
• https://vitejs.dev/guide/static-deploy.html#gitlab-pages-and-gitlab-ci