doc: update 1.12 release with RTD theme

We've replaced the "zephyr-docs" theme with a simpler read-the-docs
thema starting with the 1.13 release.  This PR retrofits the theme
changes back onto this 1.12 release and ignores attempts to use the
zephyr-docs theme controlled by the make DOC_TAGS option.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>
9 files changed