correction and minor improvements to Conventions page

[JonathanGregory]

In discussion 326 Antonio @cofinoa reported that the single-page HTML shown on the Conventions page as version 1.3 is actually 1.2. It's not exactly the same HTML file as 1.2, but it has the same text content. The single-page version 1.3 is apparently missing from the directory. I propose the simplest solution to remedy this defect, which is to remove the link, since we have the multi-page HTML and the PDF of version 1.3.

In the CF Zenodo "concept" we have all the past versions with their dates of release. The dates are in the documents in most cases, but I think it's possibly useful to show the dates for all past versions on the Conventions page as well, since we have the space available without altering the arrangement. In addition, since 1.11 we have a PDF as well as HTML of the conformance document, and it's the PDF we're depositing in Zenodo, so I've added links to the PDFs on the Conventions page too.

All these three changes are implemented by PR #564. Please could someone check and merge? [Proposals to fix defects must be approved by a member of the Conventions Committee and Standard Names Committee, or the Governance Panel. They can be merged immediately thereafter.]