Allow Nginx configuration override for try_files directive per site

[mfabing]

Currently, Herd manages the Nginx configuration automatically and does not provide a way for users to adjust directives such as try_files.
For several frameworks and CMSs (e.g., WordPress, Drupal, Laravel, custom PHP apps), it is often necessary to route requests through a front controller (index.php) when files or directories do not exist. This is typically achieved with the following directive:
try_files $uri $uri/ /index.php?q=$uri&$args;
Without the ability to modify this, developers may run into issues where URLs that should be handled by the application return 404 errors, since the request never falls back to index.php.

Example Use Case
A developer working on a Drupal site sets up a Herd environment. Drupal requires:
try_files $uri $uri/ /index.php?q=$uri&$args;
Without the ability to configure this, every path like /node/1 or /about will return a 404, making local development impractical.