Prepend APE numbers with zeroes in filenames

[pllim]

trafficstars

Example: APE1.rst will be APE00001.rst, or even APE_00001.rst

Motivation: For consistent file ordering on GitHub and ls of source checkout.

Backward compatibility: Symlink to old names or fix names in all links.

Suggested by @Cadair during Astropy Coordination Meeting 2019.

[astrofrog]

I'm 👎 on this since I think the main point of entry should be the table in the readme, which we control the order for, and it's not worth breaking links.