Bug: Local Preview "file not found" with anchor links on refresh

[greghesp]

When using local preview mode, if you click on an anchor link, change your content and reload the page, it will throw a page not found error. To resolve, if you remove the anchor link from the URL and refresh, the page loads

To Replicate

1. Create a page, with a H2 header

---
title: Test
---

Test Page

## Anchor Link

Test Anchor

2. Load the page using docs.page/preview
3. Click on the anchor link, which sets the browser URL to https://docs.page/preview/testpage#anchor-link
   4 Hit Ctrl+R

5. To resolve - remove the anchor link, ie https://docs.page/preview/testpage

The cause of the issue appears to be here: https://github.com/invertase/docs.page/blob/main/website/src/layouts/preview/utils.ts#L265-L267

Its looking for page#anchor.mdx. To confirm this, you can create another page called page#anchor.mdx in your directory. When you go through the steps above, it will load the page#anchor.mdx file rather than page.mdx