Redirect from a minor version to the latest patch version

[jpmckinney]

This will allow linking to the Extension Explorer from other documentation, without having to pin to a specific patch version of an extension.

Conceptually, each version of OCDS should refer to a minor version of an extension. For example, OCDS 1.1.4 might refer to version 4.5 of ocds_ipsum_extension (a fictional core extension). If a new patch version of ocds_ipsum_extension is released after 1.1.4, users should be led to use that patch version (e.g. 4.5.2), not the patch version that was available when 1.1.4 was released (e.g. 4.5.1). This allows bugfixing core extensions without releasing a new version of OCDS.

In practice, given that we want the extension explorer to be a static site, one option is to create redirect pages at URLs like /extensions/ipsum/latest/v4.5/, which would redirect to the latest patch version at /extensions/ipsum/v4.5.2/. To author the redirect page, we'd determine a list of minor versions (one of which would be v4.5) and look for the most recent version that matches >=v4.5,<v4.6. (Along the same lines, we could have /extensions/ipsum/latest/ to redirect to its latest version.)

[jpmckinney]

For now, the solution to open-contracting/standard#743 is to link to a specific version (through the extensionexplorerlinklist directive from sphinxcontrib-opencontracting).