doc: add HiSilicon board documentation to HTML docs

Add the README files for the HiSilicon boards to the HTML documentation.
This required a bit of reformatting.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/board/hisilicon/poplar/MAINTAINERS b/board/hisilicon/poplar/MAINTAINERS
index 9c045ea..bfd4a9b 100644
--- a/board/hisilicon/poplar/MAINTAINERS
+++ b/board/hisilicon/poplar/MAINTAINERS
@@ -3,5 +3,6 @@
 M:     Shawn Guo <shawn.guo@linaro.org>
 S:     Maintained
 F:     board/hisilicon/poplar
+F:     doc/board/hisilicon/poplar.rst
 F:     include/configs/poplar.h
 F:     configs/poplar_defconfig