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.

stagingconf.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*- #
+from __future__ import unicode_literals
+
+# Used for the "staging" branch build: a more production-like preview
+# than the bare pelicanconf.py build used for "devel" (feeds enabled,
+# absolute URLs) but still pointed at the internal dev webserver rather
+# than the real domain.
+
+import os
+import sys
+sys.path.append(os.curdir)
+from pelicanconf import *
+
+SITEURL = 'http://haemka-de.wd.haemka.lan'
+RELATIVE_URLS = False
+
+FEED_ALL_ATOM = 'feeds/all.atom.xml'
+CATEGORY_FEED_ATOM = 'feeds/{slug}.atom.xml'
+
+DELETE_OUTPUT_DIRECTORY = True