Incorporate upgrade docs into release docs

[Ed McDonagh]

There is a lot of overlap between these sections, makes sense to roll them into one.

[Ed McDonagh]

Moved content of upgrade into releasenotes and deleted upgrade. Placed new reference in install to the migration section to replace the one that was removed with the install docs revision. Removed upgrade from the index. Fixes #78.

→ <<cset 2da8b557a2e2>>

[Ed McDonagh]

Updated changes docs. Refs #78, #72, #71.

→ <<cset ae228ae13421>>