We've replaced the "zephyr-docs" theme with a simpler read-the-docs
theme starting with the 1.13 release. This PR retrofits the theme
changes back onto this 1.10 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>