Enable HSTS for GitLab Pages

[loglapa]

What changed

• Add the existing HSTS header template to both GitLab Pages nginx configurations.
• Render the configured NGINX_HSTS_MAXAGE when Pages HTTPS is enabled.
• Remove the header when Pages HTTPS or NGINX_HSTS_ENABLED is disabled.
• Clarify in the README that the existing HSTS options also apply to GitLab Pages.

Why

The main GitLab nginx virtual host honors NGINX_HSTS_ENABLED and NGINX_HSTS_MAXAGE, but the Pages virtual hosts do not contain or configure the header. Changing those options therefore has no effect on Pages responses.

The non-TLS Pages template also needs the placeholder for deployments where TLS terminates at a load balancer while GITLAB_PAGES_HTTPS=true, matching the existing main GitLab virtual host behavior.

Impact

GitLab Pages responses now use the same HSTS settings as the main GitLab virtual host, without adding new environment variables or changing the defaults.

Validation

• bash -n assets/runtime/functions
• git diff --check
• Rendered and checked four cases: native Pages HTTPS, HSTS disabled, plain HTTP, and HTTPS terminated by a proxy

Fixes #2710