Further improvements to Archive links on the discussion page

[davidhassell]

Recent changes to the discussion page were discussed and agreed at #575.

These were good, but further to this, I'd like to propose linking the Trac archive directly from the discussion page, rather than indirectly via the "about trac" page that describes how Trac was set up and used.

The reason for this is that it is the tickets themselves that are of interest, not so much the procedure on how they were used; and final link to the Trac tickets page was not that prominent.

To address this I propose a PR #581 in which

• Data/trac.html is linked directly from discussion.md
• about-trac.md is linked from Data/trac.html (thereby keeping the note of historical interest)
• Copied some of the introductory text from about-trac.md to Data/trac.html

[JonathanGregory]

I agree. Thanks for the PR, @davidhassell. It looks good to me.