DOCS: Added documentation for 1.8.0 pre-release 2 (#1785)

* docs WIP

* docs WIP

* whitespace fixes to make yamato happy

* continued docs work

* more docs work

* small docs corrections

* fixing svg/png test failure issues

* Remove unused docs file

* Fixes for broken in-page anchor links

* removed unused images

* more link fixing

* more link fixes

* more broken link fixing

* added svg source file

* removed link to missing API doc item InputSystem.actions - must fix later

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Co-authored-by: Håkan Sidenvall <hakan.sidenvall@unity3d.com>
Co-authored-by: James McGill <jamesmcgill@users.noreply.github.com>
Co-authored-by: James McGill <james.mcgill@unity3d.com>

Author: 3 people