Consider adding stable links without a full version number

[korydraughn]

One thing I was thinking about while working on irods/irods_client_http_api#493 is that it would be nice to have a stable link to the latest / highest-major versioned release at docs.irods.org. That would keep us from needing to update links in docs pointing to explicit versions of the iRODS docs.

For example, a stable link could be:

• docs.irods.org/latest -> docs.irods.org/5.1.0
• docs.irods.org/5 -> docs.irods.org/5.0.2
• docs.irods.org/5.0 -> docs.irods.org/5.0.2
• docs.irods.org/5.1 -> docs.irods.org/5.1.0
• docs.irods.org/4.3 -> docs.irods.org/4.3.5

[trel]

we used to have a /latest. not sure where that went.