[VOL-5009] - Update release notes

[VOL-5061]

overview/releases.rst
---------------------
  o Correct start/end release dates for v2.10-v2.12 to reflect reality.

release_notes/voltha_2.12.rst
-----------------------------
  o Populate the onos apps grid with versioned artifact and VCS urls.
  o Component kafka the only holdout

howto/release/checklist.rst
howto/release/components/index.rst
howto/release/index.rst
howto/release/voltha-docs/index.rst
howto/release/voltha-docs/release_notes.rst
howto/release/branching/
howto/release/components/igmpproxy/
howto/release/components/kafka/
howto/release/components/maclearner/
howto/release/components/pppoeagent/
howto/release/voltha-docs/release_history.rst
---------------------------------------------
  o Add more random content and links for release dependencies and tasks.

Change-Id: I31338488a4e626a97435429a36e200d54cf6267e
diff --git a/.gitignore b/.gitignore
index 06371ed..60ccb38 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,7 @@
 # -*- makefile -*-
 
 # build related
+__pycache__/
 .venv/
 venv_docs
 doc_venv