Web ext 8 (#1842)

* web-ext 8 updates

* removed Testing in Firefox 48

* What new section, indicator icons, update section

* Review feedback updates

Co-authored-by: Simeon Vincent <[email protected]>

* API version doc changes and general copy edit (WIP)

* Move V7 to new page - various v8 improvements (WIP)

* Reference edit competed

* removed redundant sign --id  and links fix

* Partial update to guide, removed unused version images

* Guide reorg and content link updates

* Complete additional nav for guide

* v8 commands to nav, v7 page display fix

* Update getting-started-with-web-ext.md

last_updated updated

* Updates for metadata, config files, and misc edits.

* Suggestions from review

Co-authored-by: Simeon Vincent <[email protected]>

* config code example fixes

* ID setting update

* Revise last updated date

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Co-authored-by: Simeon Vincent <[email protected]>