docs(changelog): changelog for lts-v2.8.13 release

Generated with the command:
npm run release -- --release-as 2.8.13

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I225aa9d62da21ec6fa8c5c4a04491a5a2a73d39a
diff --git a/docs/conf.py b/docs/conf.py
index 3ef0cc1..f8ae6cb 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -14,8 +14,8 @@
 
 project = "Trusted Firmware-A"
 author = "Trusted Firmware-A contributors"
-version = "2.8.12"
-release = "2.8.12"
+version = "2.8.13"
+release = "2.8.13"
 
 # -- General configuration ---------------------------------------------------