docs: align Pebrel identity and contribution guidance

Use the public project name consistently while retaining compatibility
identifiers and existing download names. Publish the contribution,
architecture and translation contracts with their enforcement limits.
Synchronize the bilingual unreleased changelog and release notes.

INSTALL.md

@@ -1,4 +1,8 @@
-# Installing Nebula
+# Installing Pebrel
+
+Pebrel retains the `Kuddev/nebula` repository, `nebula` command, and existing
+package identifiers for compatibility. Historical download filenames below
+are unchanged; do not rename installed files or configuration directories.
 
 ## Linux Preview