Add unversioned redirect to API docs

[1ec5]

Added a redirect from http://project-osrm.org/docs/api/ to the latest API documentation, currently http://project-osrm.org/docs/v5.10.0/api/. This redirect will need to be updated every time a new release is published.

/cc @daniel-j-h @danpat

[daniel-j-h]

Wait you can use redirects with github pages? I thought it would only host the static content.

[danpat]

Ur a wizard @1ec5, good idea.

[1ec5]

This doesn’t seem to work: http://project-osrm.org/docs/api/ is just a blank page with no redirect. 😞 I took this code from a redirect in www.mapbox.com, but it occurs to me now that that site is published by a tool other than GitHub Pages. Our two options, then, are either this jekyll redirect syntax or a <meta> redirect.

[1ec5]

Take two in #11.