Redirect old *.md links to *.html in Firebase config

Since we now serve rendered HTML, the file extension has to change.

In addition, output files now use a flat directory structure; the
filenames in this change correspond to the "permalink" values at the top
of each file. Our tech writers recommend this flat
directory layout as it means we can rearrange documents in a
hierarchical site outline without having to maintain the hierarchy in
the file tree, avoiding the need to physically move the files.

Change-Id: I338752597c8d97e11fe291826bf5db4e7e794243
1 file changed