mkdocs: Fix handling of root-level pages in convert_mkdocs_links.py

If a markdown page is located at the root level of the Open IoT SDK,
`{page_dir}/{link}` wrongly evaluates to `/{link}` because `{page_dir}`
is an empty string. This causes the existence check to fail for the
destination file.

Fix this by using `os.path.join()` which correctly gives `{link}`
without a preceding `/`.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>