Align contribution process document

The contribution process document has some out of date sections, tweak these to make it aligned to current expectations:

• Secure Boot document is now not a design document but an architectural document. Remove the reference as it's no longer helpful. Point to Concepts section instead.
• Designs is a section that's being removed. Point to Architecture and Guides for current implementations.
• We no longer tag releases manually, remove that as example of manual process.
• There's much less need for OBS accounts make this clearer.

Signed-off-by: Martyn Welch martyn.welch@collabora.com