Update static caching rewrite rules for IIS

[duncanmcclean]

In #1334, we updated the documentation around static caching rewrite rules to workaround various edge cases with query parameters, live preview and non-GET requests.

The updated rewrite rules essentially work like this:

• When the request is not a GET request
  • It hits Statamic / index.php to handle the request.
• When the URL includes the live-preview query parameter
  • It hits Statamic / index.php to handle the request.
• Otherwise...
  • When the page exists in the cache, serve from the static directory.
  • Otherwise, it hits Statamic / index.php to handle the request.

We didn't update the rewrite rules for IIS in #1334 because we don't have access to an IIS server to test updated rules on. If anyone has access to an IIS server and is able to test & contribute, that'd be great!