Doc update

[caspervdw]

@danielballan @nkeim @tacaswell I pushed trackpy v0.2.4 docs and changed the folder names to exactly resemble the GH tags. For the next release of trackpy, the docs will be automatically generated under the appropriate repo/tag/ folder.

For a new release, only the redirect in /stable would need to be changed manually.

[nkeim]

Fantastic! It's wonderful to have the v0.2.4 docs accessible. There may be people out there still using it. Keeping /stable under manual control is probably a good thing. Many thanks for paving the way for future releases.