doc: external_content: exclude backup files

For most people, this should be only an optimization.

For Emacs TRAMP (ssh) users, this fixes the following build failure when
a file is not saved yet. This build failure happens because TRAMP
creates backup files on the local host and seems to use deliberately
broken symlinks as "bookmarks" to those local files.

file snippets/.#index.rst

 snippets/.#index.rst: broken symbolic link
                       to userid@hostname.intel.com.966265369654

```
Traceback
=========

 Traceback (most recent call last):
   File "/usr/lib/python3.13/site-packages/sphinx/events.py", in emit
     results.append(listener.handler(self.app, *args))
                    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
   File " doc/_extensions/zephyr/external_content.py", in sync_contents
     shutil.copy(src, dst)
     ~~~~~~~~~~~^^^^^^^^^^
   File "/usr/lib/python3.13/shutil.py", line 428, in copy
     copyfile(src, dst, follow_symlinks=follow_symlinks)
     ~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/usr/lib/python3.13/shutil.py", line 260, in copyfile
     with open(src, 'rb') as fsrc:
          ~~~~^^^^^^^^^^^
 FileNotFoundError: [Errno 2] No such file or directory:
                      '___/zephyr/snippets/.#index.rst'
```

Just for the record, these backup files were already discussed in commit
bab7df9f (".gitignore: do not ignore .\#* files")

Signed-off-by: Marc Herbert <marc.herbert@intel.com>