Update Modus themes to their version 1.2.0

Update Modus themes to their version 1.2.0

* doc/misc/modus-themes.org: Add new version of the manual, with
changes to markup and references to the latest state of the project.

* etc/themes/modus-vivendi-theme.el:
* etc/themes/modus-operandi-theme.el: Provide updated version of each
theme, which expands the contents of 'modus-themes.el' (bug#45068).

* etc/themes/modus-themes.el: Add new supportive file.  This is where
theme data, functions, and face definitions are defined.