0000-TEMPLATE: add Documentation section

[elliefm]

Adds a Documentation section to the changes template file. This will hopefully:

• prompt us to find and update/create documentation as part of our development work
• make it easy for the person writing release notes months later to cross-reference that documentation

[dilyanpalauzov]

The next/ from the path can be stripped, as changes/ so far only contained the next/ directory and all essential information was beneath next/.