docs: use "dev" for non-release builds

The bug is this: If a build is done off of (say) 'main', the version coming
from 'git describe' will be something like 1.32.2.postXXXX. We previously
dropped the ".post*" suffix because it confused the spellchecker but that
just leaves the misleading "1.32.2" string. It's misleading because that's
an actual released version that does *not* contain a lot of the content
in 'main'.

So just use "dev" instead, to make it extra explicit that the docs refer
to an unreleased version of the software.

docs/conf.py

@@ -35,7 +35,7 @@
 release = rockcraft.__version__
 if ".post" in release:
     # The commit hash in the dev release version confuses the spellchecker
-    release = release[0 : release.find(".post")]
+    release = "dev"
 
 # region Configuration for canonical-sphinx
 ogp_site_url = "https://canonical-rockcraft.readthedocs-hosted.com/"