Spec page rework suggestions

[chalin]

Originally posted by @jku in #105 (comment) and #105 (comment)

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Looks nice.

On content/en/spec.md:

• is the idea that spec maintainers should update this page too when they make a specification release? This feels like unnecessary process. I would suggest replacing the list with:
  • Large button to current TUF specification
  • A bit of additional info like: "Specification is maintained at https://github.com/theupdateframework/specification/, full list of releases is available in ..."
• Currently the link for list of releases /specification/list/ is broken: should be /specification/

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Also header for the "spec" tab seems wrong -- it's got a large image that looks a bit placeholdery. Or is this intended?

Compare partial screenshot from docs and spec tab: ...

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Tasks

• Fix redirect to spec list - Redirect fix for /specification/list #108
• Propose a rework / reorg of the spec page (?)