Rebuild GDC site as a vibe.d server, converting all pages to markdown.

Rather than using diet templates, it functions in the following way:
-  Receives a request object for GET /foobar
-  Translates /foobar into a path of where the markdown file is expected to be.
```
'/'         => /views/index.md
'/foo'      => /views/foo.md
'/foo/bar/' => /views/foo/bar.md
```
-  Checks if the translated path exists as a file and proceeds to render the page in the topdown order of:
   - Template header (templates/header.inc)
   - Markdown script (views/foobar.md)
   - Template footer (templates/footer.inc)
-  Sends the built page back to the response object.

The benefit of this being that templates can be loaded onto the server dynamically and without requiring a restart of the entire service to make a change.
As of writing, the memory consumption and build time was > 50% reduction for *only* four templates generated instead in CTFE.

The only time when a site rebuild would be required is for fixing any of the todo listed items in src/app.d.