first draft technical board/code of conduct
typo, minor changes
Added list of retired core developers
Merge remote-tracking branch 'origin/technical-board' into support/3.0.x
Merge branch 'support/3.0.x' into docs-restructure2
More documentation moves to appropriate sections

* a lot of material has been moved from /topics to /reference and
  /how_to
* redundant sections (particularly navigation placeholders) removed
* /user section removed from index for now