docs: v23 gui updates (#535)

* docs: update linux install with mnemonic backup info

* docs: add mnemonic backup info to windows install

* docs: add mnemonic backup info for macos

* docs: add core qt proposal details

* chore: fix typo

* chore: remove non-existent dash central tool

* chore: update mac install images

* docs: reorganize proposal create

* docs: add missing images

Author: thephez