Add devel/staging staged-publishing flow, rename default branch to main

- .gitea/workflows/build-deploy.yml: trigger on [main, staging, devel];
  build step picks publishconf.py (main) / stagingconf.py (staging) /
  bare pelicanconf.py (devel, same as a plain local build); deploy
  step already had prod-vs-test branching, updated its ref check from
  master to main to match the repo's renamed default branch.
- stagingconf.py: new, prod-like preview config (feeds, absolute URLs)
  pointed at the internal dev webserver instead of the real domain.
- pelicanconf.py: SITEURL set to the dev webserver address (was ''),
  used by both stagingconf.py and bare devel builds.

pelicanconf.py

@@ -4,7 +4,7 @@ from __future__ import unicode_literals
 
 AUTHOR = 'haemka'
 SITENAME = '[hɐəmkɑ]'
-SITEURL = ''
+SITEURL = 'http://haemka-de.wd.haemka.lan'
 
 TIMEZONE = 'Europe/Berlin'