doc: replace UTF-8 chars

Some our Zephyr tools don't like seeing UTF-8 characters, as reported in
issue #4131) so a quick scan and replace for UTF-8 characters in .rst,
.h, and Kconfig files using "file --mime-encoding" (excluding the /ext
folders) finds these files to tweak.

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