docs: Add a page for known issues

It's common that people running old releases (for example, because of
their Linux distribution version) run into issues that have already been
fixed.

It can be convenient to have a list of the most common known issues and,
when available, their workarounds.

This patch creates the documentation page for them, currently empty. It
will be filled in subsequent patches.

.mkdocs.yml

@@ -33,4 +33,5 @@ nav:
     - sec-levels.md
     - tests.md
     - relnotes.md
+    - knownissues.md
 

docs/knownissues.md

@@ -0,0 +1,11 @@
+
+# Known issues
+
+This file contains a list of the most common known issues, and the release
+range affected. It can be useful for people running older versions, to
+identify problems (and workarounds) faster.
+
+Entries are eventually be purged once their affected versions become uncommon,
+to prevent confusion.
+
+