docs: clean up astrolabe references after extraction

Remove astrolabe-specific docs and sections that belong in the
astrolabe repo. Update remaining references to point to the
astrolabe repo where appropriate.

- Fix .gitmodules SSH → HTTPS URL for astrolabe submodule
- Remove bump-version.yml stale "astrolabe" scope comment
- Delete blog-introducing-astrolabe.md (moved to astrolabe repo)
- Remove "Astrolabe Background Token Refresh" section from auth-flows.md
- Replace "Astrolabe User Setup" section in authentication.md with link
- Remove "Astrolabe Internal URL" section from configuration.md
- Remove "Webhook Presets (via Astrolabe UI)" from webhook guide

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitmodules

@@ -6,4 +6,4 @@
 	url = https://github.com/cbcoutinho/notes
 [submodule "third_party/astrolabe"]
 	path = third_party/astrolabe
-	url = git@github.com:cbcoutinho/astrolabe.git
+	url = https://github.com/cbcoutinho/astrolabe