We should publish documentation for older release versions in addition to latest

[cdcapano]

Right now you can only access documentation online for the current master version of pycbc. This can cause issues if changes occurred to master since the latest release. For example, as described in Issue #2502, the version of pycbc_inference on master gets the low frequency cutoff to use for the likelihood from the configuration file. The example configuration files (which can be downloaded from the documentation page) reflect this. However, in the current release version, the low frequency cutoff is set on the command line. This means that if a user installs pycbc using pip, then tries to run pycbc inference using the examples documented online, they'll get an error.

If docs pages existed for each version, then the user would know (or at least could figure out) that they should look at the version of the docs corresponding to the version of pycbc that they have. This would be similar to how numpy/scipy work.