doc: board: Convert README.qemu-mips to reST

Convert plain text documentation to reStructuredText format and add
it to Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/board/emulation/index.rst b/doc/board/emulation/index.rst
index 7537179..1adefee 100644
--- a/doc/board/emulation/index.rst
+++ b/doc/board/emulation/index.rst
@@ -7,5 +7,6 @@
    :maxdepth: 2
 
    qemu-arm
+   qemu-mips
    qemu-riscv
    qemu-x86