Add links to the API documentation to the downloads page (rebased onto dev_5_0)



This is the same as gh-34 but rebased onto dev_5_0.

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The API documentation is now synced to `downloads.openmicroscopy.org/{omero,bio-formats}/{$version}/api` as part of the release job process. 
This commit adds some links to this API documentation from the corresponding downloads page.

/cc @hflynn