Add a suitable .ruby-version

The doc site currently uses some old Jekyll plugins that only
support Ruby version 2.

The .ruby-version file works with `rvm` and `rbenv` to force the
use of a suitable Ruby version. Version 2.7.8 is the last version
of Ruby 2, which is now past end-of-life.

Even for folks who don't use `rvm` or `rbenv`, the .ruby-version
file is a useful indicator of which version of Ruby to use.

Signed-off-by: Birgit Brecknell <[email protected]>

.ruby-version

@@ -0,0 +1 @@
+2.7.8