net: miiphybb: Convert documentation to rst

Convert the current miiphybb documentation to rst. Rename
the README.bitbangMII to bitbangmii.rst in the process.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index d9f2a83..c907f8c 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -9,6 +9,7 @@
 .. toctree::
    :maxdepth: 1
 
+   bitbangmii
    board_best_practices
    codingstyle
    designprinciples