mkdocs: enforce sufficiently new version, and use mkdocs-exclude.

We want to use the mkdocs-exclude plugin, which lets us exclude
particular files from the output directory.  But plugins aren't
available in the debian-stable version of mkdocs, so ensure that we're
running a sufficiently new version.  If we aren't, gracefully just skip
building the documentation.

docs/requirements.txt

@@ -0,0 +1,2 @@
+mkdocs>=1.0.4
+mkdocs-exclude